Topic: Exhaust / Jetting question

[SKTP]

Here goes...

I rebuilt my bike and now have a 4-into-1 pipe.

Everything else is stock, engine/carb-wise.

I am still in the break in period of newly rebuilt engine and my last daily driver was a Ducati Monster...that being said;

It seems pretty hard to get much faster than 60mph.

The question: is it typical for a 4-into-1 to reduce a bike's top-speed. It feels great in 3rd gear around 40mph...

I am also considering that I am just babying the bike (I know I am suppose to during the break-in) and the Ducati got me use to a totally different powerband...

[Black 750K8]

Here goes...

I rebuilt my bike and now have a 4-into-1 pipe.

Everything else is stock, engine/carb-wise.

I am still in the break in period of newly rebuilt engine and my last daily driver was a Ducati Monster...that being said;

It seems pretty hard to get much faster than 60mph.

The question: is it typical for a 4-into-1 to reduce a bike's top-speed. It feels great in 3rd gear around 40mph...

I am also considering that I am just babying the bike (I know I am suppose to during the break-in) and the Ducati got me use to a totally different powerband...

Not so sure. How many hours on the motor and others will want to know explain how you have ran it so far and how long? RPMs load time how many runs?

[SKTP]

I'm 170 miles in...I have pretty substantial oil leaking going on (I did not seal the pucks correctly) and will be pulling the motor at 500miles to address this

I did not put the stock gauges on the bike so I cannot currently know the actual RPMS. I might rig up  something temporary with the stock tach to find out how I am driving RPM-wise...

I do have a bicycle-computer/digital speedo and I have GPS tracked myself and I have not broken 60 yet

* pre engine rebuild I had the bike up to, and easily, over 100mph

* pre engine rebuild the bike took well over 5 minutes to warm up (it would stall and stall and stall)...now it starts right up and is ready to go in a bout a minute

[fdbrat]

Did you use a different pipe after rebuild?

I'd check floats and your timing. Take nothing for granted. Pros screw up too.

[goldarrow]

Need more facts/info about the bike

[martin99]

Everything everyone else said, plus-

The exhaust change may have leaned things out. A lean condition will rob you of power like you wouldn't believe, and may explain why it now gets hot more quickly. This is not a good thing. It is normal for your bike to take five minutes or so to warm up.

[SKTP]

Ok here goes:

- the bike is a 1978 CB750K
- I bought it for $400 bucks running and drove it for 2.5 year as a daily. Eventually the tranny stopped working so I rebuilt the entire bike. Everything has been cleaned and all gaskets and rings have been replaced along with a bunch of cosmetic stuff
- Stock engine and carb-wise (other than the pipes)
- This is my first engine rebuild
- I left the carbs alone, as they had been cleaned and worked great
- I did not rebuild the valves, as they seemed to work fine (The bike ran well everyday and I could easily keep up with LA Highway traffic)

- I now have a triple-A 4 into 1 pipe
- I have put 170 miles on the bike
- working out various demons (see my thread if you like. I have documented almost everything) > http://forums.sohc4.net/index.php?topic=68423.0

- The bike runs well overall and I am planning on getting it to the 500 mile break in suggestion before I rip the engine out again to deal with a 5th gear issue and an oil leak

- This issue: It seems to have lost its top-end speed

Old pipes:

New pipes:

[flybox1]

Since you started a new thread, and so we don't have to dig through your build thread for the info....
Current Jetting? 
I see a stock airbox...stock paper filter or oiled K&N?

Based on what ive read during your build, I dont think your exhaust is that far off from mine, and you shouldn't be too far off from the jetting in my signature.

[Jerry Rxman Griffin aka MuthaF'er]

Crazy ass pipes!

40mph in 3rd....?? Mine would wind to 100mph in 3rd... (around 10K) What's your rpm? That will not equate to much in 5th. Irrelavent somewhat to the scheme of things. How does it wind out in 3rd?

My suggestion is to re-install your original pipes and see what it does then go from there. That is considering you did not make any other changes especially to the carbs.

It looks nice 

[SKTP]

I am guessing that the jetting is Stock

I have never changed it and it worked well (other than a slow warm-up) when the pipes were stock

I could cruise at 70-90mph all day long

EDIT * I gave my stock pipes to Ricky Racer...and I am still only 170 miles in on a rebuild with new rings. How much danger is there in winding it up RPM-wise at this point?

[flybox1]

Stock setup was lean from the factory.
I can only assume your exhaust leans it out even more.
what do your plugs say?

[Jerry Rxman Griffin aka MuthaF'er]

Wind it up easily without load. Initially you want to insure nothing breaks like rings etc. You may be beyond that point at 170 miles. Once nothing breaks then you can do more. After that you're seating/breaking in your rings to the fresh hone and a little more should be beneficial. If you're afraid to at this point then give it a few more miles.

Since you no longer have your stock pipes you have no option. You need to read your plugs regardless. I like to start with top end (easiest since no jet overlap) then mid range. You can always increase the mains one size and see how it reacts. Just don't make more than 1 carb change at a time. Read your plugs after each change!

If you're not sure about this it's time to learn.

How much baffleing does that bowl of spahetti have? Quiet or loud? You could actually have MORE resistance with that 4 into 1 than your stock 4 pipes since they all choke down into one. Kinda depends on back pressure too since only 1 cylinder fires at a time.

[SKTP]

Thanks everyone...Yes, I have a lot to learn.

How long does it take an average person to pull the carbs and change jets? Is this a 15 minute thing once you get the hang of it?

I will check the plugs and I will continue putting around my neighborhood

[flybox1]

I usually take my carbs off.  Easier for me as I have fat hands.  Takes more time wrestling push/pull/choke cables and airbox off/on than the actual jet change, but prob around 30 min start to finish with a piss break and a beverage in there somewhere 

[SKTP]

How much baffleing does that bowl of spahetti have? Quiet or loud? You could actually have MORE resistance with that 4 into 1 than your stock 4 pipes since they all choke down into one. Kinda depends on back pressure too since only 1 cylinder fires at a time.

Sorry missed this...those pipes are surprisingly quiet. You can see straight through the tailpipe but there's a "cheese grater" type wall the diameter of the small-end of the cone all the way down to the large end...if that makes sense? Would running it with out the tailpipe be a good test idea? I know that would be very loud...but might be good for comparison and a noise ticket?

[Jerry Rxman Griffin aka MuthaF'er]

I don't take my carbs off unless/until I get to the needles. Changing the mains is easy compared to the needles. I haven't opened the pumper carbs though. Anyone correct me if I'm off base. Do you have to take pumpers off and apart to do the needles? For the mains drain the bowls. Remove the bowls. Remove the main jet holder along with the main jet. There should be a tool in your kit that fits it. Replace the main jet in the holder. Reverse the procedure and you're done.

Flybox is a whiz if he can get it done in 30 minutes. A rooky will probably take much longer.

Remember, reading plugs needs to be done in all 3 ranges. Putting around will take care of the slow jets only with some mid range overlap. You still need to check the plugs at mid-range rpms (which is where the needles AND the mains overlap flow) and top end rpms (which is where your main jets come into play).

[flybox1]

done it enough carb mods Jerry since i switched exhaust configs TWICE on my K8

assuming he's running PD carbs for his K8, no pumper carbs.  only #2 has the accel pump, and it needs not be touched when changing jets.  drain the bowls out the drain tubes first...
just drop the bowls if you can get your hands in...either way, straight-pull out the pilot jets.  the mains turn out of the emulsion tubes.

as stated above, main jet should be tuned first, the needle height, followed by pilot and then fuel screw. with a good plug chop following each change to verify.  PITA BTW, but it should be done.
since you're still breaking in the engine, it might be tough to get the main jet tuned if you cant sustain 5K+ rpms...yet 

again...what do your plugs tell you?

[SKTP]

Ok cool...I figured I would have to pull the carbs to do anything to them. This is good info. I need to, and will, read the manuals before moving forward...as well as askign you guys 1 billion questions.

I want this bike to run like a top!

[martin99]

Those carbs are likely to be PD42s on a '78, meaning the needles are non-adjustable and will need to be shimmed to raise them. PD 41s are adjustable. If you're new to this, I suggest you research how to do this first. A search on this forum will tell you how to go about it. Can be done with carbs on, but you need the dexterity of a neuro-surgeon and if you lose one of those little circlips you will be kicking yourself for not taking them off in the first place.

Here's a link to get you started...

http://forums.sohc4.net/index.php?topic=71245.0

One more tip - USE JIS SCREWDRIVERS!

[Jerry Rxman Griffin aka MuthaF'er]

Perhaps you could get lucky and find a willing professional carb guy in your area....... especially now that riding season is mostly gone here... that would be willing to do it. Could SEEM expensive but you'd be glad you went that way. Nothing wrong with learning it either and we'd be glad to help. Sometimes VERY difficult to diagnose unless we are there with you. However, it can seem like rocket science on your own the first go round. That is until you get it figured out. By all means eliminate all other potential issues first! Be sure the tune up is first class and your electricals are up to par.

Thanks for the input on my limited K7/8 carb knowledge Oldschool and Fly.

Does that link tell him how to ID the 41's vs the 42's before he gets started?   

[SKTP]

Thanks you to everyone for getting me started in the world of carbs and tuning...

I'm in Long Beach and I think I will take it to a garage to get a compression test and have them give me their opinion.

So far I am still impressed that I took it all completely apart and put it back together and it basically works...now to learn the "finesse" aspects

[flybox1]

I'll probably bend your ear one of these days Jerry as I get this F(2)'n engine done and in.
As for PD carb ident, this is the only way....unless you pull the needles and see they are notched.

[SKTP]

Found another link http://honda-cb750-s.456789.n3.nabble.com/How-to-1978-CB750-slide-needle-shims-td3688760.html

Searching for 77 needles now...seems like getting a set of those might be the best idea in the long run

[flybox1]

Don't bother.  I just sold a set of needles from my PD 41s. 
I like the shims. They're easy.

[cheftuskey121]

Don't bother.  I just sold a set of needles from my PD 41s. 
I like the shims. They're easy.

Slight side note, what are you using as shims? Small washers? I have a cycle x 4-2-1 and will more than likely be rejecting an shimming needles up.