Advice with jetting a high performance/dellorto copy carb.

Leighton

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Hi all,

I was hoping you can advise me a little more about getting the jetting right. I seem to have 2 speed bands where I get 4 stroking. Around half speed to about 65% max speed and at about 95% top speed to redline. The half speed one you can just slowly Accel through, and even the top end you can sort of wiggle the bike left and right and get a little boost ish every now and again.
I have the zeda80 engine, and it currently has only a 36tooth sprocket added - I have a proper mz65 in the post, but it's not arrived yet.

I bought a bunch of dellorto 1486 jets for the carb,
I started with 62-66 (they were quite pricey and I didn't want to fork out a large sum for the 60-80 set on eBay cos the shipping was crazy).
The 66 seemed the best but it got very, very, hot and I only rode it round the block twice.
Its always ran hot, the original user manual says you should be able to touch the head but you can't touch mine for more than half a second, even after driving it to work-about 7 miles and back each day.

With the 66 jet The water drips dropping from the frame were vapourising on the exhaust, 67-69.

I tried the 69 today and it seemed to sort out the 4 stroke zone at about half speed. But still got it towards redline. I only went aroud the block twice. It seemed a bit sluggish at the very low speeds, but an overall improvement from the stock by jet (which I took from the stock nt carb, cos that was better than the stock hp carbs ( - but it doesn't have a number onit).

I am worried about the heat. I don't want it to get too hot and destroy itself.
My questions are: do I keep going smaller until the 4strokig completely goes away throughout the speed range?
Is it meant to run at a much higher temperature to the standard HT carb jet?

Are there any trade offs from changing the jet? Like you'll loose some power low or top end depending if you are running a larger or smaller carb jet?

What are the symptoms for a much too small jet?
 

GreasyChris

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The 4 stroking during past throttle is the adjustment on the clip. Try raising the clip a notch. 4 stroking under wide open throttle is the jet is to big.
A leaner mixture burns hotter. A richer mixture burns cooler. Stock engine and head shouldn't get over 390°f after 1/2 of continuous riding.
 

Leighton

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I've still got the nt carb, but it leaked somewhat terribly, also the rubber sealing bit that goes around the ticker and carb body dissintergrated so I had to jam that hole with something else (a bunch of shrink tubing).
I'm using the hp carb because it doesn't leak, but I out the nt jet in the hp carb, because the hp jet was worse than the stock nt one.
Both were unmarked haha.

At the moment I'm at jet number 68 and it seems ok but does 4 stroke at top speed.
It seems a lot hotter than before though.

I read you should be able to touch the cylinder head. Def not possible... I'm just worried it's running too lean or I go too lean.

The spark plug check isn't really possible as there is no kill switch, I have just set it to not idle and stall (just, because I need to be able.to turn it off) so when it says get to the rev rage when it 4 strokes, hit the clutch and kill switch, I can't kill switch it and when I pull the clutch it doesn't immediately die - turns over for a few seconds....

What do you think?
 

Frogslayer

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That tickler hole is above the fuel line so either your float or your needle and seat valve were bad. By a cheap temp gun. Then you'll know the heat and it will be easier to jet safely.
 

Chainlube

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Definitely shouldn't do that, from everybody's readings lately the mean head temp is 380 F.

At the moment I'm at jet number 68 and it seems ok but does 4 stroke at top speed.
It seems a lot hotter than before though.

I read you should be able to touch the cylinder head. Def not possible... I'm just worried it's running too lean or I go too lean.

The spark plug check isn't really possible as there is no kill switch, I have just set it to not idle and stall (just, because I need to be able.to turn it off) so when it says get to the rev rage when it 4 strokes, hit the clutch and kill switch, I can't kill switch it and when I pull the clutch it doesn't immediately die - turns over for a few seconds....

What do you think?
 

Leighton

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Hi thanks for the pointers.

In this case I will leave the jet at #67, it currently only 4 strokes when I'm at max speed & redlining it.

I can't help but thinking it seems to be doing slightly better at max speed but low down getting upto speed it seems a bit slower.
The sort of mid range 4stroking zone seems to have gone though. It was almost like there were 2 power bands before. It would go from say 5nph to about 20 and start 4 stroking for a bit then would improve and hit another mini 4stroking band at about 27mph. I think it goes about 30mph top speed.

I've not had a very long ride on the 67 jet.
The 66 was the highest I tried before I bought the 67-69 and it ran pretty hot with that.
So maybe the 66 was the best it's going to be and I should just go for it and see how hot it actually gets.

I'm presuming the unmarked stock jet, means the engine runs cool for safety reasons. So with the correct jet, the normal average engine running temp should intact be a lot hotter than I had thought.

I do have a temp gun at my work so I can try that Monday.
Hopefully the 67 will be slightly rich enough to be perfect with the mz65.
 

Leighton

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Slight update.
I tried a bunch of jets and on the dellorto clone & I thought 66# was the best, however, when I was flooring it it did sort of splutter or 4stroke at top speed (when it screming). All the jets I've tried do this, but the others seem better getting upto speed.
If I went to a smaller one #65 or #64 it doesn't seem to change the top end at all and just gets hotter after 5 mins running.


I finally got the mz65 and tried fitting it, it didn't fit my frame and hit my front wheel. Getting to that conclusion I had to pretty much take the bike fully apart. When I was changing jets to accommodate the mz65, I accidently ripped the float and float mech off the carb. I had ridden the #67 before and it wasn't as good as the #66 but it was ok. Today when I rode into work it was going dead slow and sort of spluttering / 4stroking but from well early in the rev range on the #67 jet.
I think it only went about 15-20mph (that was with the #67 jet, but when I tried it before it wasnt as slow with the #67 jet).

I tried a few other jets today and it didn't change it reallt and I think it's probably something wrong with the carb after playing with it, specifically the float mech, perhaps I lost a little Oring or something...

I put my new racing NT carb on (my stock nt's tickler broke so brought the racing one), new fuel line (only this time I ditched the second fuel filter in the fuel line) and put the 66# jet in there.

It feels like it has most its power back, but, it now feels a little sluggish at low revs. Is this the carb difference do you think?
It doesn't seem to 4 stroke like it was with the delorto (think I broke it) but does feel slower picking up speed.
The needle has the clip in the one below the top groove.


Apart from the 4stroking how else do you tell if it's too rich?
I was reading last night about identifying the issue and the article said it might be stuttering at max speed and not 4 stroking.
Does anyone else get this, cos perhaps I'm confusing stuttering with 4 stroking.
(4 stroking seems a bit louder but slower rpm noise right)?

If it's too lean it will get very hot right? But are there any other symptoms, will it affect the power if it's running too lean, making it sluggish at all?

I can't really do the plug test as (to stop it I have to clutch it- so the motor slows down and I have the idle screw set low so when its clutched it just cuts out).

Any ideas?
 

Chainlube

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Move the clip to the middle slot, just because others have had good success with the second slot doesn't mean that is the right for yours. Check the float level and the idle speed, this may lead to a clue why it doesn't come off the line.

My 50 certainly didn't like the second slot.
 

Leighton

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Ok thanks I will try this now. Do you think it's running rich? It's not 4 stroking, just no torque.

It's like there is a delay in pulling the throttle to it actually picking up, you almost have to pedal to help it get going. But, when it gets to sort of half way rev range it f***ing rips to max speed! I dont know why I even bothered with the dellorto clone, it's nowhere near as good @ top speed as the nt and I tried jets from #63-69 on the dellorto.
 
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