my dads engine

Post by **callyspoy** » Sun Dec 06, 2009 12:29 pm

hello folks, now, some of you may remember my questions about my dads engine a few months ago.
a recap...
a +30 rebored 1098
940 big valve head
1.75 SU
LCB exhaust
longer duration cam(270 i believe)

right, he rebuilt the thing himself, and is currently breaking it in. but there is a real noticable drag at low revs, and as he is breaking it in, isn't taking it above 3000 revs(ish).
i went out with him as he was a bit miffed(he had a 1275 in there before). i agree that there is a problem low down, but once it gets to 2500ish revs you really do feel the power kick in a bit.
so my thoughts, he is adament that the timing is set right, but i am not sure. it doesn't feel like a mechanical issue low down, but more if a timing problem, does anyone have any ideas what it should be set to?
also the carb came straight off the 1275 A+ engine, so is there likely to much jetting issues? bearing in mind the head used on this 1098 now.
any help would be great as he is being a bit precious about it all and it's beginning to wind me up quite a bit!!

Post by **Peetee** » Sun Dec 06, 2009 5:01 pm

I would say it's down on low down torque because of the cam and large inlet. Having owned a 1043cc Polo I can say with confidence that the standard 1098 BMC engine has excellent torque low down. As with any engine mods, a rolling road sesh can work wonders, but a standard cam may be the only answer.

Post by **callyspoy** » Sun Dec 06, 2009 6:00 pm

hmm, interesting, and unfortunate if true!

linearaudio · Post by **linearaudio** » Sun Dec 06, 2009 7:25 pm

Bible, page 312(cam duration): "small displacement big valve engines rapidly loose (sic) low end output...... if you are using a smaller verion of engine then you have to be more conservative with duration". As the power is kicking in sharply, it certainly sounds cammy.

But.... I had a similar flat bottom with my hif carb, stretching the damper spring cured it by keeping the airflow speed up in the carb at lower revs. Worth a try!

Post by **Matt** » Mon Dec 07, 2009 9:45 am

I reckon it would be better with a 1.5" carb!

Post by **bmcecosse** » Mon Dec 07, 2009 9:53 am

Yes -the cam and the largish carb may be working against it. But also make sure the vacuum advance is working properly - and advance the ignition timing till it pinks - then back it off slightly.
Is the head 'big valve' or standard valve vesrion ? I reckon when set up properly it will fly above 2500 rpm - that's about when a 270 cam is supposed to kick in!

Post by **callyspoy** » Mon Dec 07, 2009 10:21 am

Ok, well dad has a couple of carbs that he can use, not sure if theres a 1.5 though. Nikki, downdraught weber, the norms!
It certainly does feel like it 'turns on' once it gets going. So if we get a 1.5 do we think it could negate the needto remove the cam?

Post by **callyspoy** » Mon Dec 07, 2009 10:24 am

Oh, the vac adv. Is working and it is a big valve 940 head.

Post by **Kevin** » Mon Dec 07, 2009 10:50 am

Well my warmed 1275cc runs fine with an 1 1/2 carb but it needed both the spring and needle changed when set up on a rolling road, which is the only way to make sure the set-up is correct..

Post by **callyspoy** » Mon Dec 07, 2009 10:55 am

I hear you kevin, but daddy's a bit of a penny pincher(he was born in glasgow!!) so i think a rolling road will be done as a last resort. Bless him!

Post by **bmcecosse** » Mon Dec 07, 2009 11:40 am

The 'big valve head' answers your question! With that cam he has built a flyer - it will be fine! Don't bother changing the carb - that's the beauty of SUs -the variable throat is self regulating to the airflow. May be worth fiddling with the spring though - as LA suggests, but DO try tweaking up the timing. The needle will have to be optimised later - just go with plug checks at the moment to make sure it's not running too weak.

Post by **Kevin** » Mon Dec 07, 2009 11:42 am

Well so is a large part of my family and its not called penny pinching just cautious

Post by **bmcecosse** » Mon Dec 07, 2009 11:43 am

You won't get anyone more 'cautious' than me!

Post by **callyspoy** » Mon Dec 07, 2009 11:47 am

So smaller carb and we will see. Excellent. Ill keep you posted!

Post by **callyspoy** » Mon Dec 07, 2009 12:12 pm

Sorry roy, only just saw the rest of the post. Ill get dad to do as you and la say. Cheers.