bike not revving

[brzio]

hello i have 1e40qma engine and i put 47mm boar on it 70cc chines kit and new stock crank and stock 50cc head and i have problem when revving it goes 4.5k rpm when the auto choke heats bike runs got rev 6k rpm and drops 5.5k rpm and i put 78 mian jet and puls beter and revs 6k rpm but whit stock boar 50cc revs at 8.5k to 9.5k rpm plz help what i need plus when full throtle sounds like its hitting or some thing like that sounds like problem whit detonation help  and i have pod filter and sotck carb 

[aeroxbud]

If your running a pod filter, I would expect to be using a larger main jet. Can't you put the air box back on?
Some of these cheap Chinese big bore kits have been known to make less power than the stock 50cc. The port timings are all wrong.

[brzio]

yes i have tried and gets to much fuel and spark plug gets black and before whit smaller jet had more rpm now has more torque and from the exhaust is dripping fuel mix and plus when holding 2/4 throttle runs than starts to bogging down,
i dont know but i thing i have problem whit the head maybe i has small combustion chamber and a lot of compression

[repherence2]

welcome to the nightmare of the Pod Filter.

in my experience, running a small diameter head on a larger bore is no bueno. and like you mentioned, will probably lead to detonation. you need to get a 47mm head or you need to modify your 50cc (40mm?) head because your squish area is basically fuct up because of the flat surface, the sharp 90 degree corner, and then the transition into the 50cc squish band. that area is where the heat is going to build up.


your 70cc chinese bore kit might be an "economy" replacement bore kit. the port timing on the exhaust is probably very low. if that is the case, no matter how much parts or jetting you throw at it, none of it will really make a difference because it would still be exhaust limited RPM wise.

[brzio]

i will buy 47mm head and i will try to see exhaust transfer duration and i will try to get 180 or 185.
i calculated that my exhaust transfer duration is around 168 and 169 and i will try whit degree wheel and see

[dwint]

I tried one of those ebay cylinders when i was new at this and i couldn't get it to tune with the pipe and like said had no power.If you can get a molossi from scooterco.com.They have the cheapest parts compared to others.

[brzio]

yes but i dont have 100+$ to spend on kit i will try to get it in future
and i live in europe in macedonia so here 1$ = 56 denars so like 100 $ like 6k kit and got like 6k for full top bottom build.

[repherence2]

Apr 10, 2023 6:36:02 GMT -5 brzio said:

yes but i dont have 100+$ to spend on kit i will try to get it in future
and i live in europe in macedonia so here 1$ = 56 denars so like 100 $ like 6k kit and got like 6k for full top bottom build.

if you are handy with the Dremel or any sort of hobby tool, you can raise the roof on the exhaust port.  i ported my chinese bore, and it was an animal after that.  night and day difference in power.  night and day difference in fuel consumption too

[brzio]

yes so i change the head and the bike was oky but didnt rev and i remove exhaust and i rev and i clean the exhaust was filled whit not burned oil and the bike is excellent how i wait to get new belt bc i have chinese belt and it wore like 3 mm from 16.8 to 14.6 mm and now am gona get mallosi one 17.5mm i was wandering will i get more top speed befor it was 80 km/h and now its 60 kmh ant 9k rpm so will i get 85 or more whit new belt,and i whana lower the head to get more compression and to put like restriction between the Expansion chamber and silencer i know that it will do something bc when i remove silencer bike canot go up hill it struggles but on flat road goes freely and is like 3 to 6 kmh more fast.
Any suggestion whill be appreciated
but no porting bc its my main transport and i will get new chinese kit and i will do port it.

[brzio]

i need some help i dont know where to start i will lower squish clearance on my kit and i whana port it but i dont now how what shape or i general how i wana get higher exhaust port and whill i get beter performance bc it struggles whitt 2 persons on it and dosent rev higher then 5.5k rpm so some help plz i need to know
1 what squish clearance is good and safe?
2 what kind of exhaust port map should i do?
tnx in advanced

[aeroxbud]

This is an interesting article if you want to start porting. www.pedparts.co.uk/blog/tuning-a-polini-70cc-sport-cylinder-basic-porting