Carb needs tuning?

[newman27]

I was challenged at a light today by a hopped up VW (Jetta maybe?). He definitely had some modified exhaust and who knows what else. I wasn't intending to race him but he revved his engine and pulled away fast and something in me decided "what the hell". I caught up to him and started to pull away when I heard a loud "bang" from behind me. He quickly dropped back and I hit the highway entrance ramp at about 100 MPH in fourth gear. He either backfired or worse - blew something. Score one for the Opels!

Anyway, besides relating my tale of victory this leads to a question. In third gear I was at about 5K+ RPM doing around 90MPH when the engine just went "flat". My foot was to the floor and I wasn't at redline but she wouldn't accelerate further until letting up on the accelerator and shifting into fourth. It felt very similar to hitting a rev limiter on a modern car. Any ideas on what might cause that? Is it carb jetting possibly? Note that when I installed my CAI my idle jumped from ~800 RPM to ~1,500 RPM (which I take as a good sign BTW on the CAI impact). So, I followed the instructions on the REDLINE website and adjusted the mixture and idle screws to get a good idle. Other than that, I haven't done anything else to tune the carb since adding the CAI. Any chance that has something to do with it? I guess I hadn't hit this problem before because I rarely push it (probably hadn't opened up the secondary since the CAI install). Anyway, it's not a big deal as it has no impact on my normal driving but I'm just looking for some opinions.

Thanks,

Matt

[hrcollinsjr]

Originally Posted by newman27 Anyway, besides relating my tale of victory this leads to a question. In third gear I was at about 5K+ RPM doing around 90MPH when the engine just went "flat". My foot was to the floor and I wasn't at redline but she wouldn't accelerate further until letting up on the accelerator and shifting into fourth. It felt very similar to hitting a rev limiter on a modern car. Any ideas on what might cause that? ...I'm just looking for some opinions. Thanks, Matt

Matt even though your engine is healthy, a stock engine isn't going to make a lot of power above 5K but it should still rev above it. I wouldn't waste my time revving a stock engine past 5K unless I was in 4th gear.

First get someone to check the total ignition timing. Did it make any strange noises or just stop accelerating? The rate of acceleration slows quite a bit above 5K.

A free flowing exhaust would help some and the next step would probably be a different cam.

Harold

[wrench459]

Matt
The valves floated or it leaned out. My guess would be a lack of fuel.
Go get them lead foot.

[hrcollinsjr]

Originally Posted by wrench459 Matt The valves floated or it leaned out. My guess would be a lack of fuel. Go get them lead foot.

I thought about the valves floating also, but doubt that happened. Fuel maybe but I'm guessing the main thing was that the rate of acceleration didn't match Matt's.

Harold

[wrench459]

A late model jetta will slam your doors off.
The Slowpel can only do so much against the high powered four cylinders of today. Let's not even think about going head to head with a Honda with its variable cam timing.

[hrcollinsjr]

Originally Posted by wrench459 A late model jetta will slam your doors off. The Slowpel can only do so much against the high powered four cylinders of today. Let's not even think about going head to head with a Honda with its variable cam timing.

You are sooo mean! Why don't you go ahead and tell him Santa and the Easter Bunny aren't real to!

Harold

[guyopel]

I agree that your jetting may need to be fatter on the Sec. side of the carb. and make sure your dist. has not moved and dropped your timing down . HTH
John

[newman27]

This was definitely a late model VW with shiny red paint BUT I think something happened forcing him to back off. Or, maybe he hit his rev limiter and didn't react fast enough to get it into the next gear before the backfire occurred. Still, when the Braves win on an error by the other team in the bottom of the ninth, it still puts a win in the win column .

The engine definitely went "flat" not a case of slowed acceleration, rather NO acceleration - it just sat there. There were no strange noises either. Thanks for the suggestions on the possible causes.

Matt

[guyopel]

Matt,
I forgot to add that you should check the acc. linkage for Carb. full throttle , bushings may be worn or out of position ..It's always the simple stuff
HTH
John

[Paul]

What do you have for ingition? Points & stock coil? pertronix and stock coil? An iffy spark and a lean mixture at higher revs might give that result..

[wrench459]

I hear ya Matt
You propably blinded him with orange paint out your exhaust pipe!!!
score one for the good guys
Maybe he hit the speed limiter

[hrcollinsjr]

IIRC new throttle linkage bushings and Pertronix. Matt, how high did you rev it at Dyno Days? I thought you went a little above 5K.

Harold

[newman27]

Originally Posted by guyopel I agree that your jetting may need to be fatter on the Sec. side of the carb. and make sure your dist. has not moved and dropped your timing down . HTH John

Originally Posted by guyopel Matt, I forgot to add that you should check the acc. linkage for Carb. full throttle , bushings may be worn or out of position ..It's always the simple stuff HTH John

Thanks for the tips John!

Originally Posted by Paul What do you have for ingition? Points & stock coil? pertronix and stock coil? An iffy spark and a lean mixture at higher revs might give that result..

I think my ignition is in good shape. I have the Pertronix, stock coil, a resistorless rotor (Mercury Capris), OGTS plug wires, and high quality spark plugs.

Originally Posted by wrench459 I hear ya Matt You propably blinded him with orange paint out your exhaust pipe!!! score one for the good guys Maybe he hit the speed limiter

All part of the master plan...

[gtopeler]

Sounds to me like it may be a fuel delivery issue. I had a similar problem recently. It turned out to be my cheap electronic fuel pump and a dirty fuel filter. The car ran great at normal driving conditions, but under hard acceleration the motor would “flatten out” until I lifted off the gas pedal. Just my thought……..

Ron

[newman27]

I got some good advice here and from John via PM which I plan to follow up on. While under the car today checking out all the suspension bolts, etc. (getting ready for tomorrow's GA meet) I looked up and noticed the front-most exhaust manifold bolt was hanging on by a thread. I tightened it back down but was wondering what symptoms (if any) that might contribute to. The rest of the bolts were tight BTW.

[guyopel]

Bolt might fall out and hit VW that is trying to catch up to
you !!
John

[newman27]

Originally Posted by guyopel Bolt might fall out and hit VW that is trying to catch up to you !! John

LOL - good one! BTW, I recreated the scenario today by running 3rd up to 5K RPM (after tightening down the manifold bolt) with the same result - "flatness". I need to get a bigger jet on that secondary I think .

[RallyBob]

Originally Posted by newman27 LOL - good one! BTW, I recreated the scenario today by running 3rd up to 5K RPM (after tightening down the manifold bolt) with the same result - "flatness". I need to get a bigger jet on that secondary I think .

Check the float level while you're at it. A low float will have the fuel bowl running dry quite often.

Does it flatten out if you rev it to 5000 in 1st gear? If it does flatten out in 1st as well, then it's probably jetting or ignition timing. If it revs out in 1st gear fine, then it's probably fuel delivery/starvation.

[wrench459]

I had the pleasure of following Matt's '72 GT today.
From my angle it moves out very smartly in the lower gears(no smog motor under that hood).
More that likely a fuel delivery problem.
What about the vette and the biker girl on 985 Matt on the way home? What was up with that?

[newman27]

Originally Posted by RallyBob Check the float level while you're at it. A low float will have the fuel bowl running dry quite often. Does it flatten out if you rev it to 5000 in 1st gear? If it does flatten out in 1st as well, then it's probably jetting or ignition timing. If it revs out in 1st gear fine, then it's probably fuel delivery/starvation.

Thanks Bob - I'll try that out.

Originally Posted by wrench459 I had the pleasure of following Matt's '72 GT today. From my angle it moves out very smartly in the lower gears(no smog motor under that hood). More that likely a fuel delivery problem. What about the vette and the biker girl on 985 Matt on the way home? What was up with that?

I couldn't figure that out either Dan. I assumed the vette wanted a better look at your car as he just hung out back there for some time. The biker girl I think was trying to get us to go around them while they waited for another bike that had fallen behind but I'm not sure - just a guess. It was definitely odd!