Engine floats back to idle

[blancojp]

Hello All!

Both of my Opels are now running, Stealth is not ready but the Ascona is ready to drive. During my initial test of the engine and partial tune I noticed when you accelerate, the engine floats back slowly to idle on the last 400 rpms. I took appart the complete induction side of the equation and re-assembled it but found no problems. I also replaced the distributor but the problem did not go away.

The 1.9L has a ported intake with the torker mods and a 250Cfm Holley marine two barrel carb. All cylinders fire and the sparkplugs look OK after a short run. But the problem is there, you accelerate to 2500 Rpms and the engine comes rapidly back to about 1200 and then floats back down to 800 rpms. With the exception of this problem, the engine runs quite well though not fully tuned.

Has anyone had a similar problem?

[Bill Hoffmann]

Try disconnnecting the distributor vacuum advance and see if it still happens. If that improves it, you may have too much vacuum signal at the advance port at the closed throttle position.

Bill

[blancojp]

Originally Posted by Bill Hoffmann Try disconnnecting the distributor vacuum advance and see if it still happens. If that improves it, you may have too much vacuum signal at the advance port at the closed throttle position. Bill

Sorry, forgot to say there are no vacuum lines connected yet. The carb I am using has no vacuum ports on it so the only port I have is the one on the intake manifold and it is blocked off for now. I normally do not connect the vacuum to the distributor until I have the engine running right. The car as it stands gets up to temperature and stays around 180 degrees, the timming is static due to the lack of the famous little ball and I have tripple checked the carb area for air leaks. It has a great idle at 800 Rpms and I can even go down on idle rpms to about 500 before the engine starts to shut down.

I know I am running 100cfm over the stock but I need that when the supercharger come on board. So if the problem is not solved, when the supercharger is installed it will get alot worst.

I will look at it today again and try to narrow down the problem area (if any).

[Aardvaark]

I've seen it before on engines including injected '75 Opels.

On carb equipped cars it could be a couple of things
involving timing not retracting fast enough or air not being cut off fast enough.

Weak or broken dist. mechanical advance spring(s)?
Mechanical advance mechanism binding and not completely free to return to
zero right away?

Would that carb have some sort of delay built into it for returning to full idle,
like a anti diesel valve, solenoid or dashpot?


On the '75's, it's probably the Deceleration Valve which is designed to do this for emissions reasons to allow extra air into the cylinders to more completely burn and left over raw gas. I just plug the vacuum line going to it to stop this annoying designed in trait.

[RallyBob]

The vacuum retard is designed to pull the timing back under deceleration (high vacuum). Without this hooked up you are relying on only the springs inside the distributor to pull it back.

One (light) spring has constant tension, the other heavier spring has a loose 'loop' that doesn't engage for the first few degrees...so the small lighter spring has to do all the work of returning the weights to the static position. Replace the heavy spring with the loose loop with another lighter spring with constant tension, and the problem should go away.

HTH,
Bob

[michaedo]

Could also be as simple as the throttle return spring
not having enough tension to pull the carb back to
idle.

[blancojp]

Originally Posted by RallyBob The vacuum retard is designed to pull the timing back under deceleration (high vacuum). Without this hooked up you are relying on only the springs inside the distributor to pull it back. One (light) spring has constant tension, the other heavier spring has a loose 'loop' that doesn't engage for the first few degrees...so the small lighter spring has to do all the work of returning the weights to the static position. Replace the heavy spring with the loose loop with another lighter spring with constant tension, and the problem should go away. HTH, Bob

I already checked all of that but the problem has not gone away.

1) I replaced the distributor three times, double checking the weights and the mechanicals to make sure it was OK.
2) I also deleted and added gaskets, including a 10mm spacer I had for the other engine.
3) I replaced the intake manifold just in case it was cracked.

The problem is still there no matter what I do. My next step is to install a 32/36 Ford/Webber/Holley I have in house to see what the reaction is. I will however be installing the carb backwards due to the manifold requirements.
If this does not cure the problem then it will be time to yank out the engine and use one of the other motors I have in stock.

Interesting how something so simple can be complicated at times.

[First opel 1981]

Originally Posted by blancojp I already checked all of that but the problem has not gone away. 1) I replaced the distributor three times, double checking the weights and the mechanicals to make sure it was OK. 2) I also deleted and added gaskets, including a 10mm spacer I had for the other engine. 3) I replaced the intake manifold just in case it was cracked. The problem is still there no matter what I do. My next step is to install a 32/36 Ford/Webber/Holley I have in house to see what the reaction is. I will however be installing the carb backwards due to the manifold requirements. If this does not cure the problem then it will be time to yank out the engine and use one of the other motors I have in stock. Interesting how something so simple can be complicated at times.

Changing an engine because of a 400rpm hesitation on return to idle sounds a little drastic, especially when you finally discover a small hole in the diaphram of your brake booster.

[blancojp]

Originally Posted by First opel 1981 Changing an engine because of a 400rpm hesitation on return to idle sounds a little drastic, especially when you finally discover a small hole in the diaphram of your brake booster.

Brake booster is not currently in the circuit, everything is plugged and there are no vacuum connections at all.

I believe the only logical step at this time would be to replace the carb.

[First opel 1981]

Originally Posted by blancojp Brake booster is not currently in the circuit, everything is plugged and there are no vacuum connections at all. I believe the only logical step at this time would be to replace the carb.

Replacing the carb isn't a bad step but have you thought that it could be that the problem is there are no vacuum lines attached that could be the problem?

If I have a good, functioning, smoke free engine that would idle as low as 500, I wouldn't pull it unless I was upgrading.

My opinion only, of course.

[RallyBob]

Originally Posted by blancojp I already checked all of that but the problem has not gone away. 1) I replaced the distributor three times, double checking the weights and the mechanicals to make sure it was OK. 2) I also deleted and added gaskets, including a 10mm spacer I had for the other engine. 3) I replaced the intake manifold just in case it was cracked.

Okay, but if you didn't change the second spring OR hook up the vacuum retard, the problem will still be there no matter how many different distributors you put in!

[wrench459]

JB
Every four cylinder engine I've ever worked on has a base idle(stock) of 900 rpm's. I checked the FSM but the spec ain't there.
I'm running very fast timing with the breaker plate locked down. No vac can at all. With this setup it does hang for a few seconds until it drops down from 1500 rpms to my base idle of 1250.

[blancojp]

Originally Posted by RallyBob The vacuum retard is designed to pull the timing back under deceleration (high vacuum). Without this hooked up you are relying on only the springs inside the distributor to pull it back. One (light) spring has constant tension, the other heavier spring has a loose 'loop' that doesn't engage for the first few degrees...so the small lighter spring has to do all the work of returning the weights to the static position. Replace the heavy spring with the loose loop with another lighter spring with constant tension, and the problem should go away. HTH, Bob

I replaced the spring as stated above and the problem was still there. It did get a bit better since it now went slowly from 1050Rpms back to idle.

I then double checked the carb I used and there it was. The carb is a 500Cfm instead of the 250cfm unit I should have gotten. All of the Holley two barrel carbs I have look just about the same, very minor differences. I replaced the carb with a Holley 5200, which I mounted reversed due to the primary being on the wrong side. The problem went away and you could tell the difference just by the initial start. The idle can now be as low as 450Rpms and it during my initial check, the acceleration is constant throughout the range with no hesitation.

Now the throttle snaps up in Rpms and then down to idle with no floating. I still have to statically tune the engine since the little ball is missing from the flywheel.

Some of the stats on the engine are as follows:
1) Oil pressure - 4.5 bar cold / 1.67 bar hot (3.2 bar @ 3500 Rpm)
2) Coolant temp - 178 degrees maintained
3) Idle set to 800 Rpms
4) Vacuum 17.6 @ idle

Thanks guys, tunning will start today.

[hrcollinsjr]

Originally Posted by blancojp I replaced the spring as stated above and the problem was still there. It did get a bit better since it now went slowly from 1050Rpms back to idle.

IIRC idle specs for a M/T is 850 to 900rpm.

I then double checked the carb I used and there it was. The carb is a 500Cfm instead of the 250cfm unit I should have gotten. All of the Holley two barrel carbs I have look just about the same, very minor differences.

I believe there were only two common sizes for the Holley carbs you are referring to. Probably the most common is the 500CFM also the cheapest. The smaller 350CFM would be better suited for an Opel street engine, IMHO.

Harold

[blancojp]

Originally Posted by hrcollinsjr IIRC idle specs for a M/T is 850 to 900rpm. I believe there were only two common sizes for the Holley carbs you are referring to. Probably the most common is the 500CFM also the cheapest. The smaller 350CFM would be better suited for an Opel street engine, IMHO. Harold

The one I was supposed to use is a marine version which has less than 300cfm's. For some reason, the box had the right markings but the wrong carburator inside.

Either way, with the Holley/Webber 5200 it runs quite well so I believe the carb will stay for now.

[hrcollinsjr]

[quote=blancojp;192386]

Originally Posted by hrcollinsjr I believe there were only two common sizes for the Holley carbs you are referring to. Probably the most common is the 500CFM also the cheapest. The smaller 350CFM would be better suited for an Opel street engine, IMHO. Harold[/QUOTE The one I was supposed to use is a marine version which has less than 300cfm's.

I forgot that you had mentioned that it was a marine version.

Harold