BlownCorona Posted July 17, 2014 Share Posted July 17, 2014 Quick general engine/bike carb question so in the coming weeks ill be moving my corona up north with me again where i can finish tuning my new engine and get a wof and all that fun! ever since putting the gsxr bike carbs on the engine can been pretty shakey at idle and smooths out when rpms are brought up above about 1200-1500, seeing as bike carbs are still a bit of a black magic conversion i havnt been ablle to find much info online. my first presumption and im hoping this is all it is. is that its because i havnt balanced the carbs yet, and the cylinders are 'fighting' against each other. anything else that would cause this? im hoping the fact that it smooths out as soon as revs are increased rules out crank imbalances and other scary things can be seen quite easily in the 2nd start video, so i didnt bother taking a new video. Quote Link to comment Share on other sites More sharing options...
peteretep Posted July 17, 2014 Share Posted July 17, 2014 theres no black magic, just tune it right remembering that these dont have a choke like a normal car, so either up the revs for when its cold and accept a higher warm engine rpm at idle or keep the revs lower and accept a shakey cold engine and normal warm engine revs. Also check the timing, and balance between carbs Quote Link to comment Share on other sites More sharing options...
CaMpylobacter Posted July 17, 2014 Share Posted July 17, 2014 balance them and make sure your timing is set right as per the service specs to begin with. you can probably get away with doing the timing by ear. I reckon you need to chuck your idle a little higher from the sounds. It may idle a bit poos anyway due to having pretty hooj carbs compared to stock. prolly doesn't have a whole lot of manifold vacuum anymore Quote Link to comment Share on other sites More sharing options...
BlownCorona Posted July 17, 2014 Author Share Posted July 17, 2014 fair enough, its also got a mild cam, so thats not helping the idle situation haha. vacuum is dismal at best. it will actually tick over as low as 500rpm (is set to 1000 though) without any concern apart from the engine shaking, it just doesnt seem like a good thing to be happening. in the coming weeks i will balance and fine tune them and set a final idle speed. but you guys seem to agree its nothing serious underlying? Quote Link to comment Share on other sites More sharing options...
CaMpylobacter Posted July 17, 2014 Share Posted July 17, 2014 if it were me i wouldn't be tooooo worried. make sure you give your tappet clearances a once over jus tin case you've got a couple of tight ones. Quote Link to comment Share on other sites More sharing options...
phatt20 Posted July 17, 2014 Share Posted July 17, 2014 sounds like vacuum leak or unbalanced carbs to me, being cv type they idle very smooth even with large cams/cold starts in my experience. they almost never go out of balance unless someone has been fiddling /had carbs split etc too. not to the point of making it idle on two cyl ish like it sounds. Quote Link to comment Share on other sites More sharing options...
Steelies Posted July 17, 2014 Share Posted July 17, 2014 my cbr carbs (keihin 36mm cv) have a choke/cold start assist. the two biggest improvements in smoothness came from - chasing down all the leaks (pressure test manifold and a million fiddling with the hose joints, x 10000) - balancing the carbs, though only by ear/listening tube. this made it so much better i didnt even bother with a manometer, just drove to nats. after nats the carbs were pretty much hanging off and it would run on two-ish below 1500 rpm then be happy as larry above that. Quote Link to comment Share on other sites More sharing options...
BlownCorona Posted July 17, 2014 Author Share Posted July 17, 2014 someone had deffinatly been fiddling with them, perhaps for racebike application as alot of the setting screws were waaaayyyy outside their normal ranges. but ive set them all to the standard setting ready to be fine tuned after ballancing (and then probably rebalance after tune) i think ive chased down all the leaks using the engine start spray method, but i could be mistaken, im also not 100% confident in the brake booster, as sometimes when you hit the breaks the engine does nothing, and some times it will affect it quite alot. Quote Link to comment Share on other sites More sharing options...
BlownCorona Posted July 17, 2014 Author Share Posted July 17, 2014 PS retep. the black magic joke should be taken with a grain of salt. im sure you've done your fair share of research, and come across all the people that are just adamant they will never work on a car/don't understand how carbs work. and it generally slows down the spread of the conversion over tried and tested webbers/dells cheers for the confirmation guys, and i shall update my project thread soon with the last little bits before wof and i might even be able to bring it for a hoon to hanmer 1 Quote Link to comment Share on other sites More sharing options...
Jase Posted July 21, 2014 Share Posted July 21, 2014 when you look at the bike they came from can you work out how much airflow it was consuming and the airspeed it was generating at idle to drag the air in at idle past the fuel to get efficient mixing. eg 1200cc bike will consume 600cc of air per revolution of the crank, at 1000rpm its going to consume 10000cc of air per second or 21.1888cfm - which would be 255cfm @ 12000rpm so then your 2000cc car will consume 1000cc of air per revolution , at 1000rpm its consuming 1000000cc/s or 35.315cfm- or 282cfm @8000rpm now from what I can remember most flatslides run a venturi to spray the fuel up from the jet when the needle is lifted, maybe this needs to be resized to match the increased airflow demanded of the carbs and also the increased volume of fuel from the jets required to achieve the correct mix for the larger engine. Also of note is most bike carbs run with an airbox extremely well, the airbox's I've seen always have relatively small inlets, could the small inlet be aiding and increasing airspeed down each carb through its restriction. Also watching your vid could the airflow from your fan be disturbing/upsetting the carbs? as normally they'd draw a relatively undisturbed supply from the airbox only really effected by inlet pulse up and down the airbox and supply and demand variation due to throttle opening Quote Link to comment Share on other sites More sharing options...
BlownCorona Posted August 5, 2014 Author Share Posted August 5, 2014 hey i only just saw this reply. all extremely good points actually. i had only ever worked out if the carbs would be able to flow enough for ~6000rpm, ive not even considered flow rates at idle and if i need to modify anything. the car is getting very close to being trailered up here, so i can run thought all sugestions. Quote Link to comment Share on other sites More sharing options...
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