THUNDERZ Posted July 8, 2004 Share Posted July 8, 2004 I did a search with no luck on this. I have a misfire at mid to high rpm's. I have a 1980 L28 with triple dellorto carbs, .020 bore, medium cam , ported head with a header to 2.25 inch exhaust. My ignition is 1980 stock. This is in a 74 260z. It has a holley blue fuel pump and regulator set to 5 psi. with a return flow configuration. I have played with jetting and every combonation doesnt affect the misfire. I just rebuilt carbs and checked float levels which are right on. I checked valve lash today and it seems tighter than what the book said it should be but this is the way it came from the shop that built it and I havent had this problem until I rebuilt carbs and put in a new fuel pump. I cant imagine that valve lash can get tighter on its own right? My timing is set at 15 dbtdc static. The engine is running fantastic at idle and about a quarter to half way through the rpm's. At this point it stumbles a little to where it interferes with the power. Its like a series of little back fires in the carbs, not loud but little muffled pops. Any ideas would be appreciated. Quote Link to comment Share on other sites More sharing options...
Guest RCNSC Posted July 9, 2004 Share Posted July 9, 2004 Hmmmmmmmm, sounds like you need to get rid of those carbs. Ill take em off your hands for you. J/K Quote Link to comment Share on other sites More sharing options...
THUNDERZ Posted July 9, 2004 Author Share Posted July 9, 2004 Hmmmmmmmm, sounds like you need to get rid of those carbs. Ill take em off your hands for you. J/K If I wasnt so frustrated by this problem I would laugh at your comment Quote Link to comment Share on other sites More sharing options...
zxgts-4 Posted July 9, 2004 Share Posted July 9, 2004 ThunderZ aren't cars a fun hobby! Check for any blockages in fuel line, tank pickup etc. Check vacuum and mechanical advances in distributer. If you have correct tools set valve clearances to spec, seats wear and valves stretch which is why FSM recommends periodic adjustment. As to any issue with your actual carb setup, someone else with experience of this setup will need to chip in. Check that you didn't get sold a "dud" pump ie that it does pump enough fuel. Good luck Neil. Quote Link to comment Share on other sites More sharing options...
Guest RCNSC Posted July 9, 2004 Share Posted July 9, 2004 Hmmmmmmmm' date=' sounds like you need to get rid of those carbs. Ill take em off your hands for you. J/K [/quote'] If I wasnt so frustrated by this problem I would laugh at your comment Well at least your Z runs! 8) Sorry I can't be of any help. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 9, 2004 Share Posted July 9, 2004 You haven't mentioned balancing the carbs. I assume you did that along with all your other stuff... Quote Link to comment Share on other sites More sharing options...
THUNDERZ Posted July 9, 2004 Author Share Posted July 9, 2004 yes they are balanced... I realized I wasnt checking valve lash with correct #'s. Will do that in A.M. with cam specs. Quote Link to comment Share on other sites More sharing options...
ezzzzzzz Posted October 27, 2004 Share Posted October 27, 2004 Well, let's try to look at this logically. Why did you perform any of this work? Why did you replace the fuel pump? If the older pump is working or you have a viable replacement then you need to try that first. It becomes more difficult to resolve before and after problems if multiple changes have been made without verification of each step. Why did you rebuild the carbs? Did you use blower style gaskets (plastic) or paper? how did you clean the carbs? Is the misfire a constant stumble? Is there backfiring in the exhaust (indicates ignition problem) at mid/high rpm's? Was anything else disconnected in the process of work? Is there a chance that a plug wire was pulled and damaged or not reconnected properly? I seriously doubt that valve lash is the problem. It sounds like a fuel delivery problem to me though. A wideband O2 sensor installed in the exhaust with the LED meter installed can tell you a world of info about what is happening? These are not terrribly expensive and can be used to curve fuel delivery for best performance. Quote Link to comment Share on other sites More sharing options...
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