Slight Hesitation above 7000RPM

Went for a drive last night. Car ran well but had a slight hesitation above 7000RPM. Pulled strong in all gears but above 7000 it would stumble a little bit.

I KNOW that the carbs need to be rebuilt so trying to diagnose or fix this may be a futile effort.

I plan on doing the carbs this fall when the rains come to Houston.

Oh, '77 308GTB.

could be point bounce if you don't have the 4 spring (bands) poinrs. could also be incorrect dwell angle.

I was wondering where that extra 5HP went to.

Seriously, I hope you get it sorted out.

When do you want to run again?


JL

Hi.......

Above 7,000 RPM ?? Have you modified the engine ?? Perhaps either fuel starvation or ignition failure or both.

Valve spring "bounce" ?? What is the rev limit ??

Frank....23005

Through experimentation I found that if the carb main jets are marginal in size, engine power will noticeably flatten out at high rpm. This isn't a "stumble", but an actual power stall. Going up one size in main jets corrects this.

On my 78 it was worse, and the 125 mains were slowly raised (using chassis dyno and exhaust monitoring) to 140's.

I believe the stock 77 main jet is either a 130 or 135, so going up a size would most likely help.

While Bill brings up a good point,
one thing to remember is that even though you may have a 7700 RPM Red Line.
Every 308 with a stock cam that I have seen dyno numbers on,
shows that the cam falls off at around 6850 RPM.

"I was wondering where that extra 5HP went to"

I was maintaining a safe distance, not to mention that my tires SUCK and I didn't know the roads as well as you do. Is that corner 90° or just 30°???? I also had no clue where you were going, how fast I could go, etc.etc....

We'll compare again once I get bigger tires and solve this little hesitation issue.


Back to the topic,

Dwell angle certainly could be off and the main jets are stock size. I'll try that line first since I've heard from several others they noticed a definite increase in top end grunt when they went to bigger jets.

Also it may be possible that the cams are not PERFECT but all the marks lined up when I did the belts in the same place as before I started.

If the cams were off would that not cause a problem all over the RPM range?

Also, if the dwell angle was off would that only affect the high RPM's or all across the range?

The more I think about it the more I think it may be the jets. The car is definitly better than when I picked it up. A friend helped me re-set the carbs after I did the belts and we adjusted the timing as well (8°BTDC is what I've seen recommended - at least that's what my memory tells me).

The car is also still set up with the R1/R2 points and the micro switch does still work (but that would be a idle/low RPM issue, yes?)

Amazing what richening up the mixture will do when you move from a higher altitude to the 50-feet above sea level Houston dense air climate!!!

I had the carbs set beatifully in KC and the hesitation showed up here in Houston. I turned the screws out ~1/2 turn, took it for a run and hesitation is gone. Of course, now it's a bit too rich and it makes a gurgle when I shut it off. The carbs need a rebuild and resinc. and I need to have the disb. checked as well.

Add to the list for this fall when the rains come.