Misfire through carb on throttle application help please

[IDOXLR8]

I have just finished restoration of our 1989 G reg Mini.

998 A+, +.040 pistons, 0 decked, Standard stroke, lightened and balanced crank, rods etc. Cooper S head with oversize valves, SW5 cam set at 106, standard distributor vacume advance with Petronix ignition, Initial advance set at 6 btdc, small bore LCB into RC40, Minispares intake with 1.5 SU (HS4 waxstat) AAU needle, original spring Idle mixture set using colour tune. (about 6 turns from fully seated)

I have the engine running, it starts & idles fine.

The issue is when you apply throttle, it backfires through the carb.

If you gently apply throttle it will rev fine if a little slow.

It leads me to think it is not getting enough fuel.

The tank was cleaned during rebuild, new fuel line to the pump. New mechanical pump, new lines to carb. Float is working and set properly.

I need some direction here as I am at a point where I am lost.

What are your thoughts? I need to get this thing running well enough to get it mot’d and broken in before putting it on a dyno.

Can you recommend a dyno shop near Cheltenham where I can have it tuned properly once it is running?

Cheers David

[vrooom]

Timing could be way out....

[eccles]

yup, i'd check the timing.

[guru_1071]

ditto timing

i think we have all been there..................

[love machine]

I would have thought that it would have been a pain to start if it was over advanced and if it was over retarded, the manifold would glow!

Since part throttle increases the advance, it says that spitback through the carb happens when the spark happens whilst the inlet valve is open. I would have thought that flooring it would have worked and feathering it would have made it spit. So, something else must be making it spit.

I'm going to have a think about this one as I've often wondered myself. meanwhile, any ideas? It would be easier to understand if the engine was lost spark and had big cam overlap.

I'll have a think, in the meantime, curious to hear what the experts have to say.

Get the timing gun on it and tell us what the deal was!

[Wildfire]

Sounds like a nice engine, suspiciously like the one i have. Check your timing and then check for any air leaks near the inlet manifold. also maybe try a AAA needle, that's what went well with mine.

Edited by Wildfire on Tuesday 3rd October 19:51

[Mr2Mike]

Weak mixture or very retarded ignition timing are usual suspects for spitting back through the carbs.

[cone]

You have'nt said what air filter your using , if its a K&N - with S type head your going to need a lot of fuel in there on partial throttle, have you tried filing the needle in the "cruise" section , standard needles very rarely give enough in that area. Rolling road session needed imho.

[cooperman]

Sounds like timing, unless one of the inlets is sticking slightly open. Or, could there be a cross-feed on the ignition circuit causing the thing to fire on two cylinders at once at certain times? That's not something I've ever experienced, but it would cause this if it happened. Cracked dizzy cap?

[GreenV8S]

Possibly cam timing (rather than ignition)?

[Mr2Mike]

IDOXLR8 said:

998 A+, +.040 pistons, 0 decked, Standard stroke, lightened and balanced crank, rods etc. Cooper S head with oversize valves, SW5 cam set at 106,

Has the block been pocketed to clear the valves?

[fwdracer]

I'd look at Mixture just off idle. Get the needle out and make sure it isn't bent or biased over. The waxstat HS4 needs to have everything straight & true to function properly regarding needle and jet. Spring biased stuff is less fussy.

[guru_1071]

waxstats always use a spring loaded needle, the waxstats often cook and make the cars run really rich.

i still think the problem is timing............

[OrangeD]

I know it's a bit different but a friend of mine had exactly the same problem with a 3.5 rover V8 back as soon as any load was applied we sorted the timing and it runs like a dream, there was a lot of head scratching before we got it though
I concur with the general view TIMING

[Wildfire]

Mr2Mike said:

IDOXLR8 said:

998 A+, +.040 pistons, 0 decked, Standard stroke, lightened and balanced crank, rods etc. Cooper S head with oversize valves, SW5 cam set at 106,

Has the block been pocketed to clear the valves?

I would think it would have to be. I have 35.6 x 31 in my 1040 and the block is pocketed. I have heard of trying to recess the valve seats to circumvent doing this, but I reckon it is a bit of a bodge.

[MR2Mike]

Wildfire said:

Mr2Mike said:

IDOXLR8 said:

998 A+, +.040 pistons, 0 decked, Standard stroke, lightened and balanced crank, rods etc. Cooper S head with oversize valves, SW5 cam set at 106,

Has the block been pocketed to clear the valves?

I would think it would have to be. I have 35.6 x 31 in my 1040 and the block is pocketed. I have heard of trying to recess the valve seats to circumvent doing this, but I reckon it is a bit of a bodge.

That does sound like a very horrible solution. Having to pocket the block is not ideal but makes the best of a bad job so's to speak.

Was just wondering if OP hasn't had block machined and consequently one or more valves are not quite as straight as they should be... However, with a high lift cam I'd have thought the engine would have stopped dead if this were the case.

[IDOXLR8]

Thanks Folks,

Timing is set, 6 degrees BTDC at 1,000 RPM with the vacume advance plugged. Cylinder head is a 12G295 with oversize valves. This is not a large bore head so hence no valve pockets in the block.

The general concensous is it is not getting enough fuel.

I have changed to a AAA needle last night and it is 80% better but not perfect.I am going to play around with a heaver daspot spring to see if I can get more accelerater pump action (vacume) by slowing the raising of the piston hence richening the transition.

If that goes no where I will go to a AAM Needle.

I found this needle comparison program. It is worth a play.

www.mintylamb.co.uk/suneedle/

[Mr2Mike]

IDOXLR8 said:

Timing is set, 6 degrees BTDC at 1,000 RPM with the vacume advance plugged. Cylinder head is a 12G295 with oversize valves. This is not a large bore head so hence no valve pockets in the block.

Ahh, confusion was caused as it was refered to as a Cooper S head in the opening post. This head was fitted to the Cooper (non-S) 998 (and MG1100's Midgets and Sprites).

Getting SU'd set up correctly on a modified engine is almost impossible to DIY. If you can get it to a driveable state then get it to a decent rolling road that understand SU's ASAP and you will be making the most of all the mods you have done.

[Wildfire]

Yep, mistake cleared up. Cooper S head is actually a 12G940 1275 casting, not the 295 (as Mike said). We're running a AAA on my brother's car which is a 998, with 1.5 roller rockers, 12G295, and Stage 1. Although next month it gets a lightened flywheel, competition clutch and SW5.

Nifty little page!