Sudden high revving while driving

[Bubbausa]

Hi all, have some questions and seek advice for a 250 inline 6, that has had some changes done…but not done by me.

What I have is a 71 block/engine, a year unknown HEI, a/c, a divorced choke Rochester mv monojet with a rod hooking into arm at top front for choke. It kind of works, more or less, but not real smooth. Carb looks to have been recently rebuilt. It also has a fitting to the intake manifold with a rod that connects up to carb. Running 93 octane non ethanol gas.

Recently while driving I feel the truck picking up speed/surging, so I put in the clutch & its revving at over 3k. Tap the accel pedal lightly and goes back to normal 6-700 revs. So, I disconnected the choke and that mystery rod (heat riser?) from intake. Lubed all linkage, could see nothing binding as I worked it by hand. Went for drive, same thing. Squirted carb cleaner around base gasket, no pickup of speed at idle. Went for another drive, high revs again. Air leak somewhere? Carb issue?

Timing was fine. Was wanting to change plugs and read about HEI requiring a gap of @.045. Looking AT Denso Iridium 5305 pregap set at .044. Correct plugs?

Also want to get new plug wires so I need HEI type 8mm. Any one brand better than the other? Anything else I can check on distributor or should replace?

Thanks for any help or direction you can provide.

[Steeveedee]

Could be linkage from the gas pedal to the carb is binding. Try putting on a stronger return spring just for giggles. I'd personally remove the current spring first and actuate the system by hand, feeling for resistance.

[Bubbausa]

Thanks will give it a try. Linkage from firewall out was pretty loosey goosey after some lube, but worth a shot. Thanks

Any ideas on plugs n wires?

[LockDoc]

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Not sure what year your truck actually is but here are some pictures of a '67 I6 linkage....

Make sure you have a return spring like Steven said.
.

[Rust_never_sleeps]

Assuming you secured the choke flap wide open and the fast idle cam down when you disconnected the choke, but those can cause chaos if they're allowed to flop around

[RichardJ]

Denso 5305 is a gasketed plug, generally used on pre-70 inline heads. Check to see if your heads may need tapered seat plugs. That shouldn't cause your problem. If the idle always returns to normal when NOT driving, it is probably not a HEI problem.

Your '71 could have one-of-hundred variations of the monojet. You're going to need to show some pictures or maybe several

[DSC67]

You mentioned surging while driving which makes me think of vacuum leaks or main metering jet too small - either of these would have you running too lean which will make the engine surge. That of course doesn’t explain the engine running up to 3k RPM. Just as a quick check, I’d pull a plug to check see.
In any case sounds like a fueling problem. If you always have good return on your linkage in your tests, don’t see how that could be the issue, assuming you’ve tried putting your toe under the accelerator pedal to pull it back when it does happen.

['68OrangeSunshine]

Plug wires: Any good 8mm set will do. I usually get a V8 set that's got unfinished ends and cut-to-fit and crimp on the HEI Distributor terminals. That leaves 2 wires extra. Every third set and you've got a free set of plug wires. Just try to get the same product all three times.

L6s are not usually built to close tolerances and high compression. Do you really need the 93 octane? 8:1 is the usual factory compression, 7.5:1 is not unheard of on early 250s. I told my machinist to build for 8:1 so I could use the cheapest pump gas.

My 292 came from the Factory with a Manual Choke. Which I prefer, and have retrofit other trucks with Manual Choke.

Stockcarb linkage thru Model Year 1970 is pretty much bombproof connecting to a Monojet. I had to convert to a '72-style Cable Throttle control because I had a Holley 4160 R8007, 390 CFM four barrel on an Offenhauser intake. The Aftermarket linkage I wound up with was too much of a Rube Goldberg set up, and was vulnerable to failure from small part loss, jamming, and even Carb Icing. It also would never swing open far enough to get WOT. On a 2005 rebuild, I converted to throttle actuation and threw on an Edelbrock 1405, since after 25 years and 250K miles, the Holley had problems.

Since AC Delco went to Mexico, I use NGK or Denso plugs. I try to get copper tip plugs. They are cheaper and you won't ruin them by touching up theelectrode with an ignition file. I never trust pre-gapped plugs, and set the gap on each at .045.

Surging may be caused by a intermittant vacuum leak. Hard to say from thousands of miles away.

[MARKDTN]

You can get AC Delco 946J plug wires for a '78 Camaro 250 for $23 on Rock Auto. I have always had good luck with Delco wires.

[Bubbausa]

Thanks for all the help. I got some kind of flu crud that knocked my old as* into the dirt for a couple weeks.

Been ordering a few things and plan to get out to the garage soon to chase this down.

Most likely a vacuum leak somewhere, and National Carb has a like new rebuilt Monojet that I ordered. Seems something funky with the fast idle cam and that divorced choke mess. Accel is not cable type, just monkey motion rods etc.

Cant figure if tapered or straight plug but need to pull one and see whats there when I finally get to it. Does it make a difference though?

Thanks, George

[Steeveedee]

The plugs have to be right. Neither will seat properly in the other's holes.

[Bubbausa]

I have denso 5330 plugs I ordered but was a little confused when ordering them. So that is a gasket plug, iridium, if I understand correctly for pre 70. Pulled one from the box and it is a short thread, flat at base where electrode is, with no gasket. Marked with ITF165330. Do I need to order gaskets if that is the one to use with the HEI?

Since the head is a 71 engine code number 250, is that a tapered plug? what should I order for a tapered one?

Thanks,
George

[RichardJ]

The heads are pretty much interchangeable on the 230-250 292 inline six. It doesn't mater what you think is true or what the seller claimed, there is a possibility that during the last 55 YEARS the heads could have been replaced.

Look at this thread and then take a good up close look at the plug holes on your head
https://67-72chevytrucks.com/vboard/...hlight=tapered

Forget the iridium plugs, save your money and just get the regular copper core plugs. That inline six will never know the difference

[Bubbausa]

Fixed! Multiple problems, thanks for the leads to sorting them. The linkage connecting arm at the carb was wallowed out and caused the linkage stick to over rev…but the fast idle cam slot was tweaked also…then old vaccum hose…and a broken spring on the divorced choke. It leaked at the gasket, so what the heck, was having to take the carb off anyhow to fix all that stuff, I just put a rebuilt carb, spring, new fuel filter, and new vacuum hoses. Bingo…that did it…runs like a champ.
Started right up once fuel filled carb, after it warmed up , did the minor adjustments for mix & idle, it drives like a new truck. Shout out to Bob at United Carburetor in NH 800 466-6167. Family business since early 50’s. Carb was dead on right and came with new fat gasket.
Thanks folks for all the tips.

[DSC67]

Glad it’s sorted out!

[tim_mc]

Glad you figured it out and thanks for posting the fix! I was guessing that there was some kind of binding if you can blip the pedal and it goes away:

Tap the accel pedal lightly and goes back to normal 6-700 revs.