[CJWAZZ]

Hi,
A year or so ago I installed a Holley street dominator intake manifold and Holley truck avenger 470 carburetor on my ’75 Bronco. Everything else is stock on the engine & it had no emissions equipment on it. Ever since making the change I’ve had problems with the engine hesitating and being low on power once it warms up, additionally, it will stumble and idle very poorly when I come to a stop. It does not have these problems when it is cold (choke closed). I recently checked the compression which read ~145 psi in each cylinder, I also checked manifold vacuum at right around 21” Hg at idle. I’ve made no adjustments to the carb since I took it out of the box. Does anyone have any ideas what my problem might be or if there are any other diagnostic tests I could run to try to figure this out?
Thanks.

[BroncoJoe19]

I can't walk you through how to adjust a carb, but we had an issue here not too long ago where the fuel pressure was a little high and the Holley tech suggested adjusting the float level. I'm not sure how that worked out, my suggestion was to get a fuel pressure regulator.

So if the problem has been since you made the change, I would check the fuel pressure and then talk to someone about checking the float level, then adjusting the carb fuel mixture needle valves.

Now you'll have to wait for someone who has more recently worked on carbed engines to give you some advice that you can actually put to good use.

[CJWAZZ]

Thanks for the response. After posting yesterday I found another post on here outlining a similar problem. To deal with the idle problem I drove it around until it was hot and choke plate opened and adjusted the idle mixture screws which made a huge difference. Turns out that I had to richen each screw by about a half turn, I guess that makes sense in that it idled fine when the plates were closed (less air) and badly with the plates open. This did not help with the hesitation problem so I messed with the timing and found that advancing it by about 8-10 degrees from where it was at before made a huge difference, it's running much much better now at idle and under power when hot and cold.

The only thing I'm still a bit confused about is why it previously had plenty of power when cold but no power when hot. I always assumed that if the timing was off, it would have the same power problems at all temperatures. Anyway, it's much better now so I'm happy.

[Leadsled]

CJWAZZ, on Dec 12 2008, 10:33 AM, said:

Thanks for the response. After posting yesterday I found another post on here outlining a similar problem. To deal with the idle problem I drove it around until it was hot and choke plate opened and adjusted the idle mixture screws which made a huge difference. Turns out that I had to richen each screw by about a half turn, I guess that makes sense in that it idled fine when the plates were closed (less air) and badly with the plates open. This did not help with the hesitation problem so I messed with the timing and found that advancing it by about 8-10 degrees from where it was at before made a huge difference, it's running much much better now at idle and under power when hot and cold.

The only thing I'm still a bit confused about is why it previously had plenty of power when cold but no power when hot. I always assumed that if the timing was off, it would have the same power problems at all temperatures. Anyway, it's much better now so I'm happy.

Sounds like you were starving your engine and the fuel was detonating probably when the piston was on the way down or when it was at TDC instead of on the way up like you have it now.