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wayne1978trucks

3.5 v8 high idles only when weak mixture?

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First of all guys ive looked through the forum and cant find a definite answer...

We have a 1984 110 v8 low compression engine that ran rough so we checked and fitted the following....

1. Checked compression, all around 110psi.

2. fitted new plugs, leads, coil, electronic ignition, oil and filter.

we then noticed the carbs would not balance no matter what but a previous owner had bodged the linkage and cable and fuel was passing the needle even when it wasnt running. (carbs were incorrect anyhow hif6s)

3. So we purchased a set of SU Hif44s from burlen.

We have set it up and balanced it at idle running at about 8 deg btdc and it ticks over beautiful, but when you hit the throttle it pops and farts and wouldn't pull off.

We then tried more advance 10 btdc but still no pull however it does idle higher with more advance.

We then tried to set the mixture richer and it now pulls alot better BUT the idle has shot up to about 2500rpm.

nothing i do seems to get the idle down even unscrewing the idle screws until they no longer touch the accelerator cam.

By the way Burlen say the carbs are supplied as standard with correct jets needles and springs, there are no known mods to the engine but all this testing is being done without an air filter on.

Thanks in advance for any ideas.

wayne

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Posted (edited)

You have an air leak. Setting the carbs up richer has masked it, and the high idle you're now seeing is caused by the extra air, plus an artificially rich mixture ahead of the leak allowing it to fire properly. Check the brake servo connection, various bungs that may or may not be in the inlet manifold and finally the inlet gasket itself. A can of carb cleaner sprayed around while running is a good diagnostic tool. If the revs change as you waft the spray around, you're close to the source of the leak. Just be careful not to spray too much! Listen for a hissing sound too. 

I'd get more or less this from my Strombergs if I left the brake servo connection off (or not blanked if you have no servo). No reason it would be any different on the SU's. 

Edited by lo-fi

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Also, be careful, HIF mixture screws are an oddity, in = richer, out = leaner!

But as above, I am almost certain you have an air leak.

What have you done with the engine breather ports on the carbs? A photo would tell a thousand words here.

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Rapid response thanks guys!!!

Lo-Fi Ive got no carb cleaner but have got brake cleaner....a little sniff of this drops the revs so i sprayed a little around the pipes and gaskets and the only area it has a difference in revs is at the inlet manifold to head on the LH rear port.

So I will get her stripped and new inlet gasket put in place, do you also recommend a smear of silicone on the gasket too?

Bowie both of the engine breathers are connected as per standard to the rockers with flame traps (i think thats what they are called)

Bowie i will get a pic later but dont judge me on the mess in the engine bay, the engine is being tested and run in in a donor vehicle (which is a knacker) so i dont elbow my wings to death on the csw.

One quick question, why when i advance the timing way beyond 8 degrees btdc do the revs keep climbing say up to 14 degrees? my manual says set at 7 btdc but ive heard folk say 8 is better

 

Thanks again wayne

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Also  Guys i wondered, because I have been tweaking the mixture over and over at small increments, would you suggest when ive finally got a setup that works i turn the screws back to the bridge height and back out equally just to make sure ive not gone an 1/8 further round with one of the screws?

thanks

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Sounds like you've got your culprit and found the air leak. Brake cleaner does the job fine. No need for any schmoo, if you can fit one of the newer style composite gaskets - they seal better than the tin ones anyway. The tin gasket does call for a smear of something; maybe Hylomar. Silicone is worse than nothing at all as fuel makes it swell and disintegrate. 

Timing has quite an effect on how effectively the mixture is burned. You should be testing with the vacuum advance disconnected, so once reconnected it brings the timing up to that sweet spot assuming the take off point in the carb is in the correct place (but that's for another discussion another time). Too far can increase CO emissions. Leave the timing by the book for the moment and tweak later. Get that air leak sorted and you'll have a much easier time with everything.

Definitely reset the carbs and rebalance. Have you found the "lifting the plunger" trick to check the mixture for each carb? I'll find you a link if not. 

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I don't think the HIFs have a lifting plunger, do they?

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Yeah, they're the CV type with the moving needle and dashpot. A bit different in construction to the Strombergs as the piston moves up and down in a cylinder with some kind of o-ring seal rather than a diaphragm at the top, but same principal. 

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Not looked into 'lifting plunger' etc.

Take into consideration most things my father and i have owned have been diesels.

The v8 and twin carbs are completely new to me.

I will do a forum search once back together. 

Thank you again guys.

Wayne.

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As I said, I think the plunger is only on the HS carbs, not the HIFs.

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