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Thread: Difficulty starting a fresh rebuilt engine

  1. #121
    3000 Post Club m610's Avatar
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    I found this - https://motofaction.org/mechanical-1...t-power-valve/

    It shows how to test the power valve.
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  3. #122
    3000 Post Club m610's Avatar
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    This sounds familiar - https://www.opelgt.com/forums/afterm...tml#post916426

    Like many threads here it ended without any resolution. It seems in the end Alex was chasing vacuum leaks.

    Mike

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    3000 Post Club m610's Avatar
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    Quote Originally Posted by Manta Rallier View Post
    Edit to add: Also, check the bottom of the carb on the secondary side.... there should be a stop screw for the secondary throttle plate when it is closed. Set this so that there is a very small gap between the secondary throttle plate an it's bore... this is a normal source of air into the intake manifold, and if it is too far closed, then the primary has to be set too far open.
    I just found something about this in my Weber book (Haynes 10240, p7-21). Thanks.

    Edit - I checked and it is set right. Also, it does not crack the secondary open. It just sets a gap for the engagement of the secondary's linkage.
    Last edited by m610; 6 Days Ago at 10:54 PM.

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  6. #124
    3000 Post Club m610's Avatar
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    The power valve was bad. I replaced it. No change.

    I called Alex and he can't remember how he fixed it drip problem.

    At under 800 rpm the vacuum retard line is definitely pulling 5 mm Hg. When I unplug it the engine does not change speed. When I plug it into the can the engine speeds up considerably.

    Tomorrow I'll put in a different carburetor.

    Mike
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  7. #125
    Member The Cub's Avatar
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    Quote Originally Posted by m610 View Post
    I'm following that thread, as you know.

    The article says the vacuum advance for a '71 is 7-10 degrees.

    I had to hold off until the hot part of the day passed, but now I'm out there again trying stuff.

    Per the excellent suggestion above I hooked up my Mityvac and tested.

    Vac. Advance RPM
    0 mmHg 0 degrees 750-800
    5 mm Hg 5 degrees 900
    10 mm Hg 30 degrees 1400
    15 mm Hg 30 degrees 1400

    When doing this the vacuum hoses from the carburetor and manifold were plugged. The small hose going from the manifold to the valve cover was installed.

    I watched the lever from the can as I pumped a vacuum and it stops moves a total of about 3 mm. On a 19 mm radius that comes out to 13 degrees.

    Max advance at 3500 rpm with vacuum advance disconnected was 27 degrees at 3200 rpm.

    When I connected the vacuum hoses rpm rose to 1400 with advance at 32. The vacuum on the advance hose was about 7 mm Hg at idle. I can get over 20 mm Hg with some throttle.

    Total advance was 40 degrees, vacuum plus mechanical, at over 3000 rpm.

    It seems to me mechanical advance is coming on too soon.

    Mike
    You’re absolutely right Mike, mechanical advance coming in too soon is your main problem there. The first thing that I thought is that your large return spring loop fell off, it has some play in it before it starts engaging so there’s a bit of normal slack, possibly enough to cause it to get loose. Your total numbers when all in are looking good. I tighten up the loop on that spring on mine because it was causing a drift at idle. Having a spring come loose and disengaging would cause exactly what you have going on with the timing. See if you can get that checked out first. That would be where I would start, it won’t cure the gasoline dripping out from your main Venturi but it’ll make a difference in tuning on the carburetor side.
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  8. #126
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    Quote Originally Posted by m610 View Post
    I'm pretty danged sure I have the vacuum hoses hooked up correctly. Like I noted above, they are different sizes and so go on only one way. The larger hose runs from the carburetor to the big side of the can, and the smaller hose runs from the manifold, near the booster hose outlet, to the small side of the can. There is one more hose that runs from below the booster hose point to the valve cover. Previously I had that hose plugged but it didn't seem to make a difference when hooked up.

    The idle speed screw is in a ways. I can't get the engine to idle with it any farther out. So, I rebooted and per https://www.carburetion.com/Weber/adjust.htm is set the idle screw to 1.5 turns in and the mixture screw to two turns out. I managed to start the car and keep it running, but adjusting the idle mixture screw did nothing, and the main is still dribbling gas. I even closed the mixture screw and the motor kept running.

    Could the power valve be stuck open/leaking? My understanding is that valve is supposed to open when a lot of throttle is applied and it lets fuel into the main circuit by bypassing the jets. That's why in our few dyno experiences A/F jumps to 9:1 when throttle is first applied.

    I had never heard about cracking the secondary. I'll try that when I am done typing.

    Mike
    OK, very good. Well, you are obviously getting air and/or fuel in from elsewhere to disrupt the operation of the primary idle circuit. You have 3 sources of air: primary throttle gap, secondary throttle gap, and PCV. So those can all play in to the air input side. Since these 32/36 carbs are set up for engines with PCV, that would be part of the typical air intake setup for these carbs.

    The secondary throttle blade gap to the throat at rest should be pretty small but not so that the throttle plates touch the sides of the throats. There IS an adjustment screw for this..... it is item #72 in this diagram: Weber 32/36 DGEV DGAV Diagram Just make sure it is set to a very small gap. (And I have not read up on the cam you are using to see if that is part of the situation.)

    I'm not 100% clear on what you have for a PCV function.... That also plays into the air entering the intake manifold at idle. If you have the stock hoses in the stock locations like in this link, then that function is present: http://www.opelgtforum.nl/downloads/vacuumsysteem.pdf The hose to port B on that diagram to the small port on the valve cover is the PCV function; plugging this port would cut off that source of air into the manifold.

    And you are correct on the power valve.... it only opens below a certain vacuum level, when the throttle(s) are opened far enough. Your idle vacuum levels right now are plenty high enough that it should be closed. The test in the link you provided is only a partial test; if there is a leaking gasket between the body of the carb and the top or bottom, then the vacuum source to close the power valve will be lost and it will remain open.

    See what you get with another carb. The fuel dribbling from the venturi is abnormal, and very, very probably the issue, so the reason why needs to be found. You HAVE to get the carb to where the idle mixture screw has command over the idle mixture or it will never idle right. If you timing is consistent at idle, then I would set it around 5 degrees advanced and ignore that matter for now.
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  9. #127
    Bikini Inspector Frozen Tundra GT's Avatar
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    Quote Originally Posted by Frozen Tundra GT View Post
    Due to new vacuum profile, I would use a mityvac and get a reading on your power valve and trim a couple springs so your power valve don't dribble...
    Your dribble is from power valve. You must trim a couple spring coils off. There are ciuple write ups on the forum.
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  10. #128
    Just Some Dude in Jersey The Scifi Guy's Avatar
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    I'm a huge fan of trimming the Power Valve spring. I had used a Combo-ish cam for 25 years and always had carb idle adjustment and transitioning from idle problems. I learned to always flutter the gas pedal as I took off from a stop to combat it. I did the flutter for so long that I almost can't drive a GT without doing it reflexively.

    Then I learned on this forum about low vacuum causing that power valve to open prematurely and cause "The Stumble". I removed loops of the spring 1/2 loop at a time and test drove. Each time the stumble got less and less until at 3 loops removed I had zero stumble. I could slam the pedal to the floor from idle and instantly spin tires with no stumble or bog with my automatics. That's really tough to do with an auto, but the spring trimming made it easily possible.

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  11. #129
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    Quote Originally Posted by Frozen Tundra GT View Post
    Your dribble is from power valve. You must trim a couple spring coils off. There are ciuple write ups on the forum.
    Only if the vacuum is low enough. See here, which refers to the kick in point as 14.5 inches: https://www.opelgt.com/forums/afterm...erflowing.html

    I think the OP said he had around 15-18mmHG of vacuum at idle? I presume he meant 15-18 inches of vacuum LOL. Soooo maybe on the margin of the power valve or maybe not.... That is why to fix the timing at one level and get that variable out of the picture. Again, I did not see what cam he has to make a judgement/guess on the vacuum levels he should see.

  12. #130
    3000 Post Club m610's Avatar
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    Quote Originally Posted by Manta Rallier View Post
    Only if the vacuum is low enough. See here, which refers to the kick in point as 14.5 inches: https://www.opelgt.com/forums/afterm...erflowing.html

    I think the OP said he had around 15-18mmHG of vacuum at idle? I presume he meant 15-18 inches of vacuum LOL. Soooo maybe on the margin of the power valve or maybe not.... That is why to fix the timing at one level and get that variable out of the picture. Again, I did not see what cam he has to make a judgement/guess on the vacuum levels he should see.
    Oops. You're right. Vacuum is in inches Hg.

    The motor is a stock '75 motor, stock cam, low compression pistons, stock head.

  13. #131
    3000 Post Club m610's Avatar
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    It's hot here.

    Backing up a bit, I pulled the distributor to check the advance springs. One is jiggly lose, not doing anything at all at low rpm. Is that normal?

    I don't have spare springs so I swapped the Ignitor/Fire Ball II distributor and coil back in and now am having trouble getting consistent spark. Sometimes I get none, sometimes a steady stream but with gaps. I've to a good 12 volts to the coil. Partially old cars are so much fun. At least the part I built works.

    Did I mention it was hot outside? 104°F expected soon.

    I'm cooled off now. Going back out for more.

    Mike
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  14. #132
    OpelGT.com Übermoderator kwilford's Avatar
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    Quote Originally Posted by m610 View Post
    Backing up a bit, I pulled the distributor to check the advance springs. One is jiggly lose, not doing anything at all at low rpm. Is that normal?
    Yep. One spring has "tight" coils on both ends, and the other has an extended end coil that allows some plate movement without any spring tension until the post hits the end of the loop.

    Depending on the version of distributor, this is what the springs look like from the factory. First photo is a early distributor mechanical advance. The line-up photo shows the relative size of the springs. The shiny silver springs are my home-made springs, as I wanted more gentle but more consistent advance throughout the rpm range.

    Keep in mind that the post-to-post distances and geometry are different from the older to newer design, so the shorter lighter early springs do not give a significantly different advance curve than the newer longer heavier springs. That said, the '75 springs shown in the photo are definitely heavier and have fewer coils than the springs shown on the "newer" distributor, as the advance curve DID change from year to year, with the '75 advance the least aggressive

    The final photo is a later version distributor mechanical advance with stock springs. HTH
    Attached Images Attached Images
    Last edited by kwilford; 5 Days Ago at 05:36 PM.
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    Keith Wilford
    Finishing up a bare-metal, nut & bolt rotisserie restoration of my '71 Opel GT; next up is my 28,000 mile Garage Find GT, stored since 1975, which needs a LOT of love...

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    Just a guess: Is the distributor shaft 'wobbly' with the Ignitor? If so, it can vary the gap a lot and when the gap gets large, you can lose the trigger signal.

    BTW, with stock-ish cam I would not expect to have to mess with the power valve setting assuming timing and idle speed are close to normal, vacuum ought to be high enough to be many inches above the level where the PV gets activated. 18" or a bit more is what I would expect; but it's been a long time since I looked at a stock setup!
    Last edited by Manta Rallier; 5 Days Ago at 05:36 PM.
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  16. #134
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    Quote Originally Posted by m610 View Post
    It's hot here.

    Backing up a bit, I pulled the distributor to check the advance springs. One is jiggly lose, not doing anything at all at low rpm. Is that normal?

    I don't have spare springs so I swapped the Ignitor/Fire Ball II distributor and coil back in and now am having trouble getting consistent spark. Sometimes I get none, sometimes a steady stream but with gaps. I've to a good 12 volts to the coil. Partially old cars are so much fun. At least the part I built works.

    Did I mention it was hot outside? 104°F expected soon.

    I'm cooled off now. Going back out for more.

    Mike
    As I mentioned I tightened up the larger spring loop some with mine. Don’t forget that you may have the stock clear resistive wire going to your coil. That could be effecting the intermittent spark on your Fireball set up.
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  17. #135
    3000 Post Club m610's Avatar
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    I figured out the intermittent spark thing. The trigger wire from the original distributor was grounding out. Oh, well. Lots of other connections got inspected and cleaned up. Spark is steady now.

    The can on this distributor is different. I doesn't move until it gets 5 mm Hg then it moves to its limit, snaps to its max. Difficult to compare apples and oranges here.

    Max advance is 25 degrees.

    Vacuum coming from the carburetor (advance hose) is 10-ish. Like Keith said above, it seems it should be zero since it is (supposed to be) above the throttle plate. I squirted some Brakleen into that port and some sprays up and the rest down, so I guess the throttle plate is partially blocking the hole. Maybe a venturi effect is generating the vacuum?

    Vacuum on the other line is 13-ish and it bounces more.

    The engine runs the same if I have the advance hoses plugged or open. Is that normal or does this mean I have a much bigger leak?

    I removed the power valve and plugged the hole. No difference. Gas still dribbles out the mains. I've checked the valve at the bottom of the bowl for crud and it is clean. I'll check again to make sure it isn't stuck open.

    After I let the engine cool a bit I'll swap in the carburetor I used on the Tinyvette, and on this engine last September.

    Thanks for all the help.

    Mike

    Edit: It's even hotter here now.
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  18. #136
    3000 Post Club m610's Avatar
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    Quote Originally Posted by kwilford View Post
    Yep. One spring has "tight" coils on both ends, and the other has an extended end coil that allows some plate movement without any spring tension until the post hits the end of the loop.
    Thanks. It sounds like the mechanical advance is good. But why the 30 degrees advance I am seeing? Should the mechanical advance be kicking in so early?

    Mike

  19. #137
    3000 Post Club m610's Avatar
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    Well dang.

    The power valve on Catherine's carburetor did not leak.

    So now, carburetors swapped, no difference.

    I have idle speed and mixture set for initial adjustments. Gas dribbles in from the mains.

    It has to be an ugly vacuum leak somewhere. Little has shown up when I spray Brakleen on the joint. I'll take it apart again after it cools a bit. I've got more manifold gaskets.

    It's only 97°F here now.

    Mike

  20. #138
    3000 Post Club m610's Avatar
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    The manifold is off again. I can't find any problems. The gasket faces are clean and flat and parallel.

    The mating surface on the head is clean.

    The header has tabs welded onto it, but the heights vary, so I have been shimming the bolts for the intake to make sure it is clamped shut/tight.

    I actually have experience with vacuum leaks. Early in my Tinyvette days we had a gradually worsening problem with bogging on throttle application and backfiring out the rear, LOUD, when lifting. We'd also get a little run-on. We found and eliminated those leaks then everything was great, and still is, on the Tinyvette, and the wagon, and Ron's car, and LT's. Basically every Opel I've worked on. But this one???

    I'm hesitant to reassemble it again. Nothing has changed. Maybe I need a different kind of manifold gasket. I have the metallic-looking one's Gil sells, same as is on the wagon, and Tinyvette.

    As for Catherine's distributor, I'm going to put it back together and put it in. It seems to be set up to have mechanical advance come on early, and so be it.

    I've run out of ideas. Thinking out loud.

    Manifold vacuum at idle is around 13 mm Hg. Vacuum on the advance hose is 10-12 mm Hg. That has to be a clue, but how does low vacuum pull fuel out of the mains? It did this with two different carburetors.

    The advance vacuum is probably due to throttle plate position, although it is set with the idle speed screw only 1.5 turns in.

    Timing is set at TDC for #1, without vacuum advance.

    Mike

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    Quote Originally Posted by m610 View Post
    I squirted some Brakleen into that port and some sprays up and the rest down, so I guess the throttle plate is partially blocking the hole. Maybe a venturi effect is generating the vacuum?

    Vacuum on the other line is 13-ish and it bounces more.
    The engine runs the same if I have the advance hoses plugged or open. Is that normal or does this mean I have a much bigger leak?
    I removed the power valve and plugged the hole. No difference. Gas still dribbles out the mains. I've checked the valve at the bottom of the bowl for crud and it is clean. I'll check again to make sure it isn't stuck open.
    .
    Mike, I know you’ve been through this, but that sounds like classic cam timing being off symptoms. I haven’t had this happen to me but when I suspected that my cam timing might be off someone told me about a good running test, https://www.opelgt.com/forums/member...s/79145?page=1 see post #2 or put your hand or something more sensitive over the primary and check for a pulsing effect that would indicate that your cam is off a tooth or more, mine was aligned okay with just an old stretched chain, I saw no misting or felt no pulsing in my case. HTH
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  22. #140
    3000 Post Club m610's Avatar
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    Quote Originally Posted by The Cub View Post
    Mike, I know you’ve been through this, but that sounds like classic cam timing being off symptoms. I haven’t had this happen to me but when I suspected that my cam timing might be off someone told me about a good running test, https://www.opelgt.com/forums/member...s/79145?page=1 see post #2 or put your hand or something more sensitive over the primary and check for a pulsing effect that would indicate that your cam is off a tooth or more, mine was aligned okay with just an old stretched chain, I saw no misting or felt no pulsing in my case. HTH
    I agree, that issue should be back on the table, and that sounds like a good test. I'll try it as soon as the engine is together again.

    It seems I need to start by addressing the low vacuum. I found a web page that lists many symptoms and their causes. I'll start there.

    Link: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

    Anyway, the carb and intake are off. I'll go through it again very carefully and will install using a fresh manifold gasket. Fingers crossed.

    I should be able to use my shop vac to test for blocked exhaust, right?

    Mike
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