Engine hunting after rebuild

[DaveG]

Hello all,
This is my first post although been with mmoc on and off since 1997..
Our 1969 4 door is nearing the end of a diy nut and bolt rebuild which has taken 10 years !,,,
Finally fired up the engine yesterday and he starts but runs very erratically, tends to rev up and down and hunt, each up and down cycle taking about 5 secs.
I've searched for intake leaks, have experimented with timing and carb jet settings but can't seem to smooth things out..

In desperation I'm turning to the good folk of MMOC.

The engine has always been mildly modded with 286 cam, polished ports, and 1 3/4 inch carb on a heated inlet. Mated to a 3 into 1 exhaust. The flywheel has been lightened, New Alternator conversion and new complete elec ignition and distributor fitted as part of rebuild. Engine machining and short build all done by very well respected specialist so I'm sure that's ok..
As part of the build I've had to bits and cleaned / refurbished every part of the car... Carb has been rebuilt with o/e kit from Burlen, new fuel tank and fuel line from back of car, new electric pump from BM..as I knew the original SU pump was knackered and felt an electric solid state one would be less prone to issues..(??)

I had assumed the hunting was due to air leak but can' t track anything down. Any ideas or advice on initial setup of a newly rebuilt engine would be appreciated ..
All the best
DaveG

[bmcecosse]

Sounds about right for a 286 cam....especially with a light flywheel. What capacity engine is it? And what needle in the carb. Try EP90 in the carb dashpot. And is the dizzy advance curve suitable for use with a 286 cam? You need to realise that the 1098 engine must not be revved over 6000, and the 948 and 1275 unwise to rev over 6500 - unless lightened/balanced/strengthened of course. The 286 doesn't really start to work properly till it gets to ~ 5000 rpm.... So you have a very narrow power band there.

[DaveG]

Hi BMC,
Thanks for the reply
The engines basically standard capacity - Geoff at Autosprint in Birmingham built it several years ago and it's sat under my workbench ever since ?.i think he used + 30 Pistons after the rebore.
The engine isn't just rough ( prior to the rebuild it ran with a 286 and was lumpy but not unpleasantly so..
This is more of a revving up and down the Rev range..
The needle Is a BDL - also unchanged since the car last ran.
I've tried ep 90 and also thinner oil in the dash pot but no joy ...

All the best
Dave G

[bmcecosse]

Standard what capacity? 948 ? 1098 ? If so - a 286 cam is not a wise choice. Don't get me wrong - it's a GREAT cam in a race engine where the revs can be used. Indeed it was my favourite (tried a 649 and went back to the old faithful 544). But must be revved. The engine may just be coming on and off the dizzy advance curve - but a good search for an air leak won't do any harm. Spray WD40 around the inlet joints to see if that causes more revving and/or smoke. Are the washers gripping the inlet manifold to the head holding both manifolds tight to the head? Sometimes the exhaust flanges are thicker - and the inlet flanges are not tightly held. Stepped washers may be required - or file the flanges to equal thickness.

[DaveG]

Thanks BMC
Daft of me not to say - sorry the engine is 1098..
I'll have another look at the manifolds tomorrow - I had thought the flanges looked a bit thick on the new 3:1 exhaust manifold but had thought I'd got away with it by filing down the thick washers where they impinged on the welds around the exhaust flanges - tomorrow's another day but for now we're heading to the pub to celebrate at least partial rebirth of our minor !!

To be honest I think I've bitten off more than was wise by stripping down the SU carb .... It was in a bad state but I'm not certain I've helped things by mucking about with the black art of carburettor building !! One last finding just now is that whenever I put any throttle on at all, the car stalls... But It's also very hard to keep going without choke...
Am I being thick and missing out on something obvious ?.
Dave

[bmcecosse]

Hmmm - 286 on a 1098?? It will just be coming 'on cam' when you have to change gear. The carb is too big for a 1098 - so a 1.5" carb will be better suited - use an AAA needle for starters. The SU carb is very adaptable with it's variable choke throat, but on a 1098 it will likely never be much more than half open. Could well be the manifold clamp washers causing the problem. That's where I would start. You would do MUCH better fitting a 12G940 head (ex 1275 engine) to the 1098 - but definitely not with a 286 cam since it lifts the valves too high.

[panky]

The 1 3/4" carb is a bit big but if it was running OK with it before, and discounting manifold leaks, could it be that the mixture is just set way too weak? Is it a HIF44 or a HS6?

[DaveG]

EUREKA !
There was a minute leak under the inlet manifold flange. Spotted it with the WD40 test and a little juggling with washers sorted the running out totally.
Engine now finished - now I just need to pop the wings back on, fit an interior, put the doors back together and pop them on, and refit all the glass.

Should be done by teatime
Thanks for the help BMC and Pankey
Dave

[panky]

Excellent, glad it was an easy fix