My car just started doing this, what could it be?

[Jack H.]

Re: My car just started doing this, what could it be?

I second this... Check the timing to see if the distributor's hold down clamp is a bit loose allowing the distributor to have moved and your car's spark advance profile changed.

[Duke W.]

Re: My car just started doing this, what could it be?

Ported vacuum advance is a big cause of run-on.

Duke

[Robert E.]

Re: My car just started doing this, what could it be?

Duke, On post #4, he stated it was a '68 L36

Robert

Ported vacuum advance is a big cause of run-on.

Duke

[Brandon T.]

Re: My car just started doing this, what could it be?

Plugs being too hot can act like glow plugs.

Stu Fox

What I don't understand though is everyone on this forum recommends r45xls's for the 427 so surely that can't be what suddenly caused my problem. Could be since I just put them in 3 weeks ago but most people had great luck with them thats why I went with the 5's instead of the 3's.

[Steven B.]

Re: My car just started doing this, what could it be?

We ran water or trans fluid down the Carb to clean off the carbon , some times the carbon build up was so bad you got a engine knock and people thought their rods were going. Anyone that don't remember this and using water will think I'm crazy but it workedI

I forgot about doing that! 'Did that to a guys '51 Chevy and a '57 283/210. It worked.

Brandon, check your timing. Is your timing currently set at spec?

[Dick W.]

Re: My car just started doing this, what could it be?

Typically I almost agree with old DW but I don't think 50 RPM would do squat. . JD

ChePhart, 25 rpm's could do it if you were right at the point of having problems. Go back and play like you are working on those fuel infestation units

[Steven B.]

Re: My car just started doing this, what could it be?

Quote:
Originally Posted by John DeGregory (2855)
Typically I almost agree with old DW but I don't think 50 RPM would do squat. . JD


ChePhart, 25 rpm's could do it if you were right at the point of having problems. Go back and play like you are working on those fuel infestation units
__________________
Dick Whittington


There is so much positive emotion here, you guys must be brothers. I can feel the love.

[Jim W.]

Re: My car just started doing this, what could it be?

Well back in the OLD days 60's 70's it was quite common. Poor gas causing carbon to build up on top of the pistons .
Which stayed lit . We ran water or trans fluid down the Carb to clean off the carbon , some times the carbon build up was so bad you got a engine knock and people thought their rods were going. Anyone that don't remember this and using water will think I'm crazy but it worked

I have used BG44K. one can and about two hundred miles and the carbon is gone. I suppose you run 93 octane or higher, and it's the premium fuels that tend to produce carbon build up. In my 2003 LS1 it shows itself as piston slap, whatever, I'd try a can. It's not cheap, but it works ?

[Duke W.]

Re: My car just started doing this, what could it be?

Duke, On post #4, he stated it was a '68 L36

Robert

...which is why I mentioned ported vacuum advance.

Wouldn't it be nice if guys stated the year and engine option in their first post instead of waiting for someone to ask.

Duke

[Randy G.]

Re: My car just started doing this, what could it be?

We ran water or trans fluid down the Carb to clean off the carbon , some times the carbon build up was so bad you got a engine knock and people thought their rods were going. Anyone that don't remember this and using water will think I'm crazy but it workedI

I forgot about doing that! 'Did that to a guys '51 Chevy and a '57 283/210. It worked.

Brandon, check your timing. Is your timing currently set at spec?

If you do it you better use a squirt bottle or a small bottled water with the cap on and a small hole punched in it. Don't pour water or tranny fluid from a cup because one big gulp (oops) will hydraulic the engine and bend a rod or break a piston.

[Brandon T.]

Re: My car just started doing this, what could it be?

was an easy fix tweaked idle down a hair haha..

[Stuart F.]

Re: My car just started doing this, what could it be?

Brandon;

What you learned was that the small change in idle speed changes the relationship of your primary butterflies to the various circuit ports in your carburetor. If opened too far for a higher idle speed you activate another circuit. Not knowing which carb you have, it is difficult to be more specific. But, if you study the circuit diagram for your carb, you will get a better idea of what I speak. Needless to say, the factory specified idle speed for a stock installation is well engineered with this in mind, i.e. the carb and the ignition timing. If you change one factor, such as changing to full time vacuum advance, your idle speed will likely go up. To return it back to normal, you will close down your primary butterflies and change the circuit relationship - often times leading to a hesitation or stumble because the transition port exposure is lessened.

I found this out with my L-76 1963 with an AFB. All I did was change to a VAC (B28) which was to give me better low speed vacuum control (63 was the first year with a VAC on a SHP engines and was a notoriously bad low end performer). When timing the initial mechanical advance to specs, then adding the B28 with more vacuum advance than stock I found my idle speed picked up by about 200 RPM. When I reduced it with the carb idle set screw back to normal I got a stumble/hesitation which drove me nuts. I played around with jetting, pump stroke and even float levels, and with a little study work I diagnosed what was going on. I preset my Idle speed screw on the bench to expose a perfect square opening of the transition port, then tested for the best combination of venturi clusters from what I had which basically gave me some different air bleeds to work with, and changed to a larger pump squirter and some other tweaks until I arrived at my preferred idle speed and I no longer have any hesitation or stumble.

Stu Fox

[Brandon T.]

Re: My car just started doing this, what could it be?

Brandon;

What you learned was that the small change in idle speed changes the relationship of your primary butterflies to the various circuit ports in your carburetor. If opened too far for a higher idle speed you activate another circuit. Not knowing which carb you have, it is difficult to be more specific. But, if you study the circuit diagram for your carb, you will get a better idea of what I speak. Needless to say, the factory specified idle speed for a stock installation is well engineered with this in mind, i.e. the carb and the ignition timing. If you change one factor, such as changing to full time vacuum advance, your idle speed will likely go up. To return it back to normal, you will close down your primary butterflies and change the circuit relationship - often times leading to a hesitation or stumble because the transition port exposure is lessened.

I found this out with my L-76 1963 with an AFB. All I did was change to a VAC (B28) which was to give me better low speed vacuum control (63 was the first year with a VAC on a SHP engines and was a notoriously bad low end performer). When timing the initial mechanical advance to specs, then adding the B28 with more vacuum advance than stock I found my idle speed picked up by about 200 RPM. When I reduced it with the carb idle set screw back to normal I got a stumble/hesitation which drove me nuts. I played around with jetting, pump stroke and even float levels, and with a little study work I diagnosed what was going on. I preset my Idle speed screw on the bench to expose a perfect square opening of the transition port, then tested for the best combination of venturi clusters from what I had which basically gave me some different air bleeds to work with, and changed to a larger pump squirter and some other tweaks until I arrived at my preferred idle speed and I no longer have any hesitation or stumble.

Stu Fox

thanks for the details, mine is the origianl 7028209 FG Quadrabog yeah the other week I had tweaked the idle up to around 700 idle must have been a little too high and actually after I made that tweak I noticed it climbed a little over 700..

tweaked it back down and no engine run on..finally, something easy