theremm Posted December 23, 2018 Share Posted December 23, 2018 (edited) I just installed a newly reconditioned cylinder head to my L24. The car idles nicely and revs smoothly through the whole rpm range when in neutral. Under load however, there is a flat spot between 2,500-3,000 rpm (when I try to push the car). It takes about 3-4 seconds to get through those 500rpm. The car revs freely under load either side of 2,500-3,000 otherwise. The flat spot is not as much of a problem if my foot isn't to the floor.. I have a Clive Cam '355' grind, and set the dizzy timing to 7 degrees BTDC as per the factory service manual specs. Not sure if that is appropriate timing for the new cam though? Forgot to ask MIA before the Xmas holiday break.. Prior to removing the old head, the car ran nicely and had smooth power through the rev range. Carbs are roundtop SU's. The previous head was the stock E88 with the original cam. Any thoughts on where the problem lies? Edited December 23, 2018 by theremm Quote Link to comment Share on other sites More sharing options...
C.A.R. Posted December 23, 2018 Share Posted December 23, 2018 It's a lean mixture at the rev range. I suggest a dyno tune to iron out the problem - it may require a new set of suitable needles in the Hitachi's. Quote Link to comment Share on other sites More sharing options...
RLY240 Posted December 23, 2018 Share Posted December 23, 2018 Seven degrees doesn’t sound like much timing so I would start by edging this up and see if that helps. Under light load the vacuum advance brings the timing up but under full load there is less vacuum. Depending on the distributor weights the mech advance might not come on until after 2500rpm. Roger Quote Link to comment Share on other sites More sharing options...
Administrators gav240z Posted December 23, 2018 Administrators Share Posted December 23, 2018 Are you still running N27 needles in the SU's? You can easily check. Simply remove the Domes and the piston with it, then remove the needle to check what is stamped on it for sizing. Be careful not go bend or ruin the needle when removing. If they don't come free use carb cleaner to soak them for a while. It might help. What condition are the SU's in? Quote Link to comment Share on other sites More sharing options...
theremm Posted December 25, 2018 Author Share Posted December 25, 2018 (edited) Thanks everyone for the replies, made a few adjustments yesterday and the flat spot is now gone. Interestingly the 1973 factory service manual suggests 7 degress of timing, however the 1972 FSM recommends 17 degrees for manual trans. Must be due to emissions gear for the later SUs? I thought that was a bit bizarre. I was also lucky enough to catch James from MIA on the phone who just popped into the shop on christmas eve for 5 minutes for a customer pickup. He suggested 12-14 degrees for that cam so I went with that. I made the timing adjustment initially and then went for a drive and that eliminated the flat spot completely between 2,500-3,000rpm. I then retuned the carbs and took it for another drive. That made things a little smoother however as per Locky's suggestion I think the needles are probably undersized for the new setup. From 0-3,000rpm the car accelerates smoothly however does feel like it could use a bit more fuel. Above 3,000 the thing goes like a rocket ship. Gav, not sure on the needles however the SUs are reasonably good - I did a full rebuild kit and clean on them about 4 years ago now. In the new year I am planning to put on a set of 44's i'm part way through reassembling, so I won't bother with any further work to the SUs just now. Hopefully the mikunis will help with the bottom end. Edited December 25, 2018 by theremm Quote Link to comment Share on other sites More sharing options...
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