Carb tuning issues - 82 F250, 351w

[Shran]

Kind of beating my head against a wall here... forgive the novel but I need some tech support and you will need all the information. I have an 82 F250 that has a 351w and C6. It was gifted to my kid and now I have it because it just won't run right consistently and nobody can figure out why. Here's the rundown of what was done to it:

- Prior owner: removed electronic carb & distributor, replaced with China 2150 two barrel and MSD distributor w/ Duraspark module. Very poorly installed, more to come on that
- My kid replaced the fuel pump and sending unit because it wasn't getting fuel.
- After that it lost spark. He took it to a shop and they found that it had a bad Duraspark module.
- It ran "ok" after that but he asked me to look at it. I found a complete disaster from the previous owner. There were sticks shoved into vacuum lines, wires clipped off, etc... at that point I removed about half of the smog junk and found that the carb was tuned very lean. Fixing that helped some; I also found that the timing was off by 10-15 degrees - it ran great at that point.
- He started having problems with it again about a week later. He thinks dirt in the fuel tank plugging up the carb. Confirmed, float bowl is loaded and making its way past the filter somehow. He keeps driving it and just sprays out the carb with carb cleaner ever 20-30 miles.
- I traded him trucks because he needed something reliable - it now not only runs poorly but it smokes really bad (blue smoke but smells like unburnt fuel, not oil.) Was not smoking at all before. I know him & his buddies were poking at it so I have no idea what they changed.

So yesterday I messed with it and swapped the China carb with a 2150 that I had on hand from a late 70's F150. I put a kit in it and also replaced the base and spacer gaskets. While I was in there I did an EGR delete and redid the fuel line so that I have a clear plastic filter that I can see into now to check for dirt contamination. It fired right up and ran good. It no longer smokes, exhaust is clear and smells normal. I am chalking that up to something wrong with the China carb - not sure what else it would have been.

Here's the problem I'm having: once the engine warms up, I cannot get it tuned to where it runs good. I can get it to idle smooth at about 17 inches of vacuum but it wants to idle a bit fast. If I slow it down with the throttle stop screw, it starts to run rough and vacuum falls to 13-14in. I can then add additional turns to the idle mixture screws and get back to around 17in of vacuum but the screws are out almost all the way (like 9 turns) and there is a hesitation right off idle if you blip the throttle

Additionally, regardless of where I have the carb adjusted, there is a really bad misfire at half throttle and higher that gets worse with load. It's not backfiring at all, just shaking a lot, so I am not sure if it's leaning out and wanting more fuel at higher RPM or if it's running really rich - I can't see fuel spraying out of the high speed pullover nozzles at the top of the air horn though so I suspect lean. I did recheck the timing... it's right on... and the vacuum advance is working.

Any ideas? Bad carb/something I missed? Vacuum leak seems very likely but I can't find one. Weak coil? Bad Duraspark box? Not sure what to think.

 

[alwaysFlOoReD]

I just watched this vid last night. It may help.

 

[Shran]

I just watched this vid last night. It may help.

All of that makes sense, I had assumed that it would need further small adjustments past the high vacuum mark and with the air filter housing in place. That is exactly how I tune small engines and it's basically the same concept... I did this with a 2.8 when I did a Duraspark swap on it last year and used the same process for that - turned out perfect.

Basically the problem with this one is that I can get it to idle pretty good but it misfires under load or higher RPM, or it runs OK at high RPM in park but the idle is terrible and vacuum measurement is really low. I also don't know how many turns out is too many on the idle mix screws but 9 seems like an awful lot, they are almost falling out - but it won't idle at all at any less than 4-5. The China carb was set at 6 when I removed it.

I have another carb (D3OF-EA) that is in good shape but it allegedly came off a 302 and has a 1.08 venturi. I could swap that on and see if I can tune that one better but I was hoping the one I'm using now (D7TE-AYA, if it matters) would be more suitable for a 351W since it originally came off of a 351M or 400 engine.

 

[alwaysFlOoReD]

Sorry, I can't help anymore. I never really learned carbs all that great. The only teacher I had was hotrod magazines back in the day. And hard knocks...lol

 

[85_Ranger4x4]

I start with the screws backed out two turns and go from there. Tune for max vacuum, then back it out a little bit. Try to get both screws even.

It could be a vacuum leak into the lifter valley as well.

Cap of everything vacuum and see if that helps.

My brother had a chinese carb on his VW, it never did run right. If you have something else I would for sure try it before I go ripping off intakes.

 

[rusty ol ranger]

Lightly seated and 1.5turns out is factory spec.

The off idle hesitation can be caused usually by either the accelerator pump, or the accelerator pump linkage being sticky. On my old 2150s i soak that in whatever spray lube i have laying around at every oil change.

2150s are pretty sensitive to float level adjustment, ive also had needle valves that will stick halfway open, and cause them to lean out. At 9 turns out to get it to idle it sounds lean across the board.

What i would do is spend the 40bucks and rebuild it, however the throttle shafts can wear and get sloppy in their bore, which will cause MASSIVE vacuum leaks...alot bigger then you would think. If that happens...replace the carb.

They are dirt simple to rebuild, i really think you got like i said a needle valve hanging up, or your floats outta wack, either way id just redo it all.

Ive never used a vacuum gauge to tune them, i set it at initial, tweak it till it smooths out, then set the idle speed, if it gets rough after that i tweak the screws a bit more. I know its not "proper" but ive always set the idle speed (if auto) while its in drive.

 

[Uncle Gump]

It's been awhile... but

Nine turns out on the mixture screws to get it smooth out... the idle jets are too small or you're partially plugged up in the idle circuits.

Float level being off a little low won't really affect the idle circuit to the point you need to back the mixture screws 9 turns. It might affect the mid range and certainly wide open.

Have you checked timing at 2500rpms? It should be full advanced or near it and be about 30 degrees BTDC.

Also... if he had to blow out the carb every 30 miles to remove dirt and rust from the carb... that speaks volumes. You might want to do a volume check on the fuel pump... and check the quality.

 

[Uncle Gump]

Did he put the filter sock back on the fuel pick up?

I just re read... does your inline filter have the nylon screen or a paper element?

 

[Uncle Gump]

Maybe @19Walt93 has something that will help...

 

[Shran]

I forgot to mention that I did clean the "new" carb and put a kit in it. It's not sparkling clean but there wasn't anywhere that I couldn't blow air or carb cleaner through. I may just pull it apart again and run it though my new ultrasonic cleaner. It was of unknown origin to begin with... I used it 'cuz I had it... came out of my buddy's scrap pile and it looked good enough to use.

I'll have to recheck the throttle shaft for wear. I don't remember there being any but I should have checked that closer.

Jets: IIRC they were both labeled #57.

Fuel tank/sock: I have no idea if he put the sock on the sending unit when the tank was out. He did replace the sending unit so I assume if it came with a new sock, it was installed. Those do a poor job of filtering out really fine debris though. He said him and his cousin cleaned it but the debris getting pulled out of it says that it was not cleaned very well.

Fuel filter: my new one has a paper element. He had one of the little metal ones that screw right into the carb - it was always full of really fine black crap and that stuff would always make its way past that filter and into the float bowl. I figured a clear one with a paper element would give me a better idea of how much is getting caught in the filter - so far after about 2 hours of run time there is almost none.

Idle screws: It won't even start and stay running with the screws any less than 4-5 turns out. I have to blip the throttle to keep it running if they're any tighter than that.

Float: I didn't really check this on my rebuilt carb but the bowl fills up and stays about 1/4" from the top. Unless I hear otherwise I will assume that's good enough, it's not running the bowl dry.

Timing (@Uncle Gump) - I have it set at 10 degrees BTC with the vacuum advance unplugged and I can watch it advance like it should while increasing engine RPM... the timing pointer is in a really bad spot under the alternator bracket and I can't see it real clearly but about 30 degrees sounds about right for what I can see at (guessing) 2500ish RPM - I don't have a tach handy, just increased RPM until it stopped advancing.

 

[Uncle Gump]

Well if you can see the timing advance with RPM... the centrifugal advance is at least working.

I still think there is an issue with the carb... based on your description. Also... if the idle circuit is lean... it's usually a bit lean through the progression ports all the way up to the main metering system.

 

[19Walt93]

The first thing I'd do is remove and thoroughly clean the gas tank so you don't have to keep chasing it.
No carb should need the idle mixture screws out 9 turns, something is plugged/restricted or the jets are too small. When the 351m showed up in 77 trucks it ran like crap and caused all kinds of complaints, one thing we did(no doubt illegally) to improve them was to install #57 jets like you have and a red power valve, I forget the vacuum rating. Those carbs also had what for described as a "stumble on tip in"- hesitation on easy throttle application- caused by the air bleed cam. We fixed it with a new cam from a TSB. I would start by stripping down and thoroughly cleaning your carb, then reassemble it following the instructions in the kit, adjusting everything and not skipping steps. If the problem persists try cleaning and overhauling the smaller carb, it might be a little doggy but it should run right if it's in good shape.
I'd rather clean and rebuild a 50 year old Ford carb than buy a new Chinese knock off. I don't think I've ever seen a good quality Chinese part= and I've encountered a lot of them working on old stuff.
Good Luck.

 

[Shran]

Thanks guys. I will for sure clean the tank or maybe just replace it - I was hoping my new filter would delay that step a bit until I know it runs right but that probably wasn't the right decision. I'll keep y'all updated.

Just for fun I pulled the top off the China carb, this is how much rust got caught in the float bowl in about 50 miles... the indentation in the bottom where the power valve sits is packed full.

 

[Uncle Gump]

The accelerator pump cavity i believe is the lowest spot in that carb. It's probably full of crude too.