View Full Version : HD8 carb adjustments ?

02-15-2003, 05:46 PM
Just installed HD8s and have the fuel mixture set as per instructions. The idle is fine till about 5 minutes later it drops from about 700 to 400. There does not seem to be any linkage left to spring back. I have stood by the car and pushed it at idle and seen no movement. Any other ideas to try?

HD8 on 29D engine

66 fhc
02-16-2003, 12:19 AM
Are these new or used HD8s?

02-16-2003, 02:47 AM
brand new!


02-17-2003, 11:46 AM
Your explaination of linkage springing back doesn't tell me enough to go on. But, sometimes the idle changing drastically for no apparent reason is caused by something hanging up. In many cases the hang up can be the fact that the jet is not centered and it won't allow the long needle in the middle of the carb to slide all the way to bottom. then you do all of your adjustments and then it bottoms and the adjustments are off. Another hang up will be the actual piston in the body of the carb. When you lift the piston (engine not running) with you finger in the throat of the carb and release it, the piston should drop smartly with a noteable metalic clunk when it hits bottom. Both pistons should give the near same sound. You can just unscrew your top chambers and polish the piston and inside of the champber with fine (nothing more than fine) steel wool. Do not oil them. oil on any surface inside the throat of the carb or in the piston area just traps dust and turns to varnish. If the piston is free in its chamber then hang ups may be a noncentered jet or it maybe in your linkage outside the carb. It may be your choke cables. I did an explaination of adjusting the HD8 carbs back on Feb 8, it is listed under 'BJ8 choke cables'. Review it if you like. (if those are new carbs I would hope that who ever put them together centered the jets, so look elsewhere first).
Dave, graemlins/cheers.gif

02-17-2003, 12:30 PM
Would the themostat opening (which it would do after several minutes of running) cause any changes in engine load?

Or does coolant suddenly start flowing in an area (head, manifold, etc.) that would affect the temp of the intake mix (again due to thermostat opening)?

Or is car "loading up" at idle, causing RPM drop due to slight plug fouling.

Just guessing here, since I have a "small" Healey.

02-17-2003, 01:25 PM
I have seen where then engine coming to full temperature would all of a sudden loose its ability to idle. In most cases this would happen if the jets were adjusted too rich and when cold the engine not only needs this enrichening but can handle the extra enrichening due to the jets being too rich, but at full temp it becomes too rich and starts to load the plugs and then can't idle.
Jerry, when the car idles and especially during its problem idle period can you notice a sooty exhaust. Partially black smoke.
But if the idle is relatively smooth, just loses a few hundred rpms and there is no sign of sooty exhaust, AND the 'carb piston lift test' indicates that the mixture is right then I believe there is something hanging up.
I would also disconnect and plug the vacuum line to the distributor at this point. The vacuum dashpot could be moving here when its not suppose to. We also could have vacuum leaks in the intake stream that only open up when warm.
But each time I have seen this loss of idle after a few minutes of running it is usually the piston in its chamber or the jet hanging up. Especially if the choke has been pulled 'full on' because then the jet is really pulled to its maximum travel and that is where it sticks. In this case the jet needs cleaned and centered.
Dave, graemlins/cheers.gif

02-17-2003, 06:33 PM
Excellent ideas to check. To eliminate one item, the choke cables at this time don't work so I never pull them. Something in the old cables is kinked and I need to replace those that go to the carbs.

I will check the needle center and piston return.

The piston lift test for fuel richness indicates that they are very close to correct. I drove it for about 150 miles yesterday and drives great except for a long stop light and the idle.

I sprayed carb cleaner all over the outside gaskets and intake area to see if there are any vacume leaks. Nothing increased the idle.

Thanks for the ideas. I will go back and look in the archieves under choke.