So after my long saga with my F-250(1985 460 C6)I think it is finally close to being road worthy.

However(Sorry my comma key is broken )It still has this horrible "bouncing" RPM at around 1500 under light throttle in park. Just around the gear changes while driving. You floor it though and that SOB revs up nice and smooth

So I have messed with the timing. Gave it a tune up. Fiddled with the carb(correct float level and choke adjustment helps) Yet this still happens.

This leaves one culprit in my mind. Vacuum.

I may be wrong though but if you guys have thoughts on what it may be they would be much appreciated!

 

What carb are you running? Is this engine totally stock or stock with the emissions ripped off, or emissions ripped off and a non-stock carb?

 

Engine has been reman'd from Ford. Unknown specs as far as that goes. A.I.R. pumps long gone. EGR is hooked up as far as I can tell. Car quest rebuilt Holly/Motorcraft 4180.

 

Sounds like you have a mostly stock engine, with lots of vacuum hoses still on it, which have probably been gone through(pulled on and off) during the engine change. I would start reading up on what everything does, and experiment by taking them off one at a time and see if the problem goes away. At least then you could narrow it down to one particular spot, or eliminate the vacuum lines and their components all together if none of them affect the problem.

 

When our trucks get up in years, as ours have, it's just good maintenance to go through 'em and do a complete vacuum line replacement at some point.

I've done this with both my 85 and 92. All hoses and connectors and any clamps that might be used. Also consider the vacuum check valves as part of this maintenance. Do it once, complete, and you'll not have to question your vacuum system for many years to come.

I'm not saying it's your issue, but it just makes good sense to eliminate the possibility. Makes for an easier diagnosis in the future, when other issues pop up.

Just my opinion, for what it's worth.

 

Could be lean or too much timing. Set timing to 10* for starters, 14* would be better if it'll take ti without detonating. Check for vacuum leaks or plugged passages in carb that could be leaning it out.

 

Thanks for the response guys. I also noticed that on the right rear carb mount there was vertical crack running down the EGR-carb gasket I suspect my brake booster vacuum line as well but the stuff up front is all new within the last 4 months.

Yes I know I should hook up a vacuum gauge

I'll be checking how clean the EGR and carb look when I put the new gasket on. Oh what fun taking the carburetor and all the lines and linkages off will be...

 

Wah!



Twist-ties from bread bags can help marking the things so they can be put back exactly like they were.

 

Hey that reminds me, you might have a burnt out gasket or casting underneath the carb and or the EGR spacer plate. We had a thread a couple of months ago(the Winnebago) where a guy had that problem with his 460.