Main Jet size, help please

[DazRR]

Hi all, I have a 57 road rocket which I have recently done a top end rebuild, rebore and new 8:1 pistons,  prior to doing this it had poor compression (approx 80psi) and poor ish performance, its now run in and seems to run smoothly and pulls well upto mid 50s but runs out of steam around 65 , I'm not in the habit of going much faster but it should be able to achieve a cruising speed of 70 or more, this performance is about the same as before the recent work, it has a 389 carb with a 290 main jet, I'm wondering if this is undersized and potentially hampering performance, any help or advice would be appreciated

[Triton Thrasher]

Never be shy to try a bigger jet.

If your main jet is significantly too small, 7/8 throttle opening at a reasonable rpm (4000?) will feel faster than full throttle.

While climbing a  slight slope at full throttle, close it to 7/8 throttle.  What happens?

Do you know for sure there’s enough fuel flow through the taps, filters, needle valves etc?

[DazRR]

There seems to be no performance gain going from 3/4 throttle to full just more induction roar,I've been running with both taps open so presumed there would be enough fuel, I'm not running an air filter and guess I'm looking for confirmation if my 290 main jet is too small

[Slymo]

It sounds more like a timing issue than carburation. If it was lean on the main jet rather than simply fail to increase in speed you would get a flattening out as the mixture became too lean. Check your plugs for colour and that should tell you if it is a mixture issue. Best if you do it immediately following a fast run with minimum idle time. Do you have a ATD or a manual advance setup on you mag? If an ATD I would look to see if it is advancing properly with a strobe.

[JulianS]

I would certainly experiment with larger main jets.

Found this document with some service literature, not very clear unfortunately and author not known.

[Triton Thrasher]

There seems to be no performance gain going from 3/4 throttle to full just more induction roar,I've been running with both taps open so presumed there would be enough fuel, I'm not running an air filter and guess I'm looking for confirmation if my 290 main jet is too small

Yes, you can presume anything you like.  Best of luck!

Or you could go mad and actually check.

[Clive54bsa]

DazRR, I run a #290 in my '61 Super Rocket and it runs great, #3 slide and position 3 on the needle. I mainly ride under 2000ft elevation. Since I changed cam from a 357 to a 356, the bike has run like I always wanted it to. I struggled for years running with the 357. I did have a problem with my '54 Golden Flash, where it would run all day at 55mph, but if you took it up to 60, the power would start to fade and then it would miss badly, but if I slowed to 55 it ran sweet again. It turned out to be a cracked slip-ring on the mag. I got that fixed and the problem went away. Like the old saying, most carb problems are electrical  and most electrical problems are carburetter.

[DazRR]

It sounds more like a timing issue than carburation. If it was lean on the main jet rather than simply fail to increase in speed you would get a flattening out as the mixture became too lean. Check your plugs for colour and that should tell you if it is a mixture issue. Best if you do it immediately following a fast run with minimum idle time. Do you have a ATD or a manual advance setup on you mag? If an ATD I would look to see if it is advancing properly with a strobe.

It has manual advance and is running electronic ignition, timing is correct but thank you for your thoughts.

[DazRR]

[/quote]

Yes, you can presume anything you like.  Best of luck!

Or you could go mad and actually check.
[/quote]

Good point mate, I'll do a flow test to be sure

[DazRR]

I would certainly experiment with larger main jets.

Found this document with some service literature, not very clear unfortunately and author not known.

Thanks for this mate.

[JulianS]

Some fuel taps come with a built in obstruction - note the the tap in the photo where the sealing membrane overlaps and partial obstructs the fuel passage. This is the type with screw on nylon filter,