1986 f150 300 l6

I did the non feedback/DUI swap little over two years ago. Ran great, a bit lacking in third, mostly up hill, and rand a bit rich.

I have smog testing where I live, and have watched the last 3 tests slowly climb to the fail mark. Hence my reasoning for the changes I've made.

I've just installed a YF carb from a 1970 F350. Replacing a carb from a 1983 F150.

I'll be ordering a new cat as I think its shot due to some rattling.

The Issue I'm having since the carb swap is bucking under load. If I unhook the vac line to the advance on the DUI I get no bucking. Poor power, but no bucking.

The only thing I did was change the carb, no other changes. Bolted her on and all I had to do was adjust idle a bit and all seemed well, untill the road test.

Thanks in advance, I look forward to the great advice you all provide so willingly on this great forum.

 

By DUI I assume you mean the distributor advance? If so under heavy load you wont have much vacuum to the advance anyway, so that weird. let me know

 

Its a DUI distributer with vacuum advance. Just to clarify My advance has always been hooked up to "ported" vacuum. Is this not correct??

 

I agree with David - you won't have much vacuum if the load is very high, so you shouldn't be getting much advance. In other words, unless are doing the load test at low RPM and very little throttle than the vacuum advance shouldn't make any difference.

So, I'm guessing that your vacuum advance is blown such that you have a vacuum leak. Pull the hose off the carb/manifold and suck on it to see if there is a leak. If there is no leak you can pull a vacuum and won't get any air flow.

And I would suggest ported vacuum.

 

its been hooked up to ported vacuum since I did the swap almost 2 years ago, never had an issue.

I swapped the carbs, took about 15 minutes, and now I have this issue.

I'll test the advance for leaks when I get a chance, but like I said, all "seemed" to be fince until I switched the carb.

 

I know were going crazy with the DUI, bucking under heavy load? powervalve, low float level, just trying to figure this out, pedal to the floor no advance, no vacuum, unplug the vacuum same thing but no bucking?

 

The new carb is quite lean. I tested a similar, if not the same carb, that 1986F150six put on his truck, and found the air/fuel ratio to be very lean. So any air leak will cause problems with the way the engine runs. But, the original carb wouldn't have been that lean so a leak via a blown vacuum advance wouldn't have been noticed.

 

Is the EGR hooked up?

 

Ok I'll target the advance and see if its leaking.

EGR Is blocked off.

I chose this carb due to its leaner charictaristics, as I was running so rich with the other carb, likely ruining my cats and would surely fail my smog test.
Where can I find the specs to check my float?

Yes with the advance un-hooked I can drive, bout the same as before I changed the carb. With the advance hooked up, it wont allow and input from the gas pedal without bucking.

Thanks for your thoughts, please keep them coming.
I'll try and check advance late tonight.

 

sounds like as soon as the DUI gets that vacuum (ported) we get the problem. just humor me and hook your advance up to constant vacuum, see how she runs

 

yea thought about doing that as well, I'll report my findings.

 

Constant vacuum will make no difference. All it will do is to put vacuum on it at idle, which means if the advance is working your idle RPM who'll go way up. And if it isn't working you'll have to enrichen the idle mix to keep it running right. But from idle on the vacuum will be exactly the same as with ported.

 

He is saying as soon as he touches the throttle it bucks, so as soon as vacuum touches the diaghram its bucking, too much advance? when the advance moves is it grounding something out in the distributor? trying to narrow down the problem! I had that plenty of times wires chafed and when the advance plate moved caused all kinds of problems. I cant see it, or hear it im guessing

 

If it is when he touches the throttle then I agree it might be chaffed wires. But I didn't get that from what he said. Perhaps I misunderstood.

 

Hmmm? The carburetor on my truck, from a 1970 F350, threw me a curve when installed. There is but one vacuum port and that is where I connected the vacuum advance for my Duraspark distributor. It just did not run right. A vacuum gauge was connected to the port and @ idle [650-700 RPMs], the vacuum was 5" of Hg. I had expected 0". When revved to higher RPMs, the vacuum climbed to ~11" Hg and would go no higher. Even when quickly releasing the throttle and expecting the vacuum to spike @ 20-25" Hg, what was seen was maybe 13-15".

Someone on the forum said the port is likely connected to venturi vacuum and pointed out that Ford sometimes used this signal to operate a solenoid which controlled emission related components [EGR?].

Anyway, with the vacuum advance connected to this port, the vacuum advance basically never worked.

Mine is connected to manifold vacuum and was that way when it was worked on @ the OK GTG back in September. Once carburetion and timing is adjusted, this set-up can work quite well!