78 Head Swap Issue's

[JerrodR]

So last Summer I made the unfortunate mistake of allowing a Buddy to take My 78 K on a "Quick Cruise". He Returned 2 Hour's later with a blown Head Gasket and a Warped Head. FF to this passed Summer I snagged a Replacement Head off eBay and Swapped the Original out. The Replacement is Functioning Perfect Mechanically, but I quickly Realize it's Running super Lean. I have a bit of Tuning experience so I got to Work. I Replaced the Accelerator Pump, and worked My way up to a #135 Main Jet and the damn thing is still Lean.. At #125 it would Run Great under 4k, but as soon as it got into the Main Jet it would Lean out and fall on it's Face. No idea what's going on, any advice is Welcome.

 

[JerrodR]

One thing ive Considered is that I possibly Purchased a much earlier Head and it's not Playing very nice with the Pumper Carb's. But that's just a Guess..

 

[wwsummers]

One thing ive Considered is that I possibly Purchased a much earlier Head and it's not Playing very nice with the Pumper Carb's. But that's just a Guess..

http://www.hondachopper.com/garage/69-78_model_id_chart/69-78_model_id_chart.html

 

[dirtdigger]

Wouldnt make a difference. I have used the stock carbs on 970cc motors, just gotta jet for what the motor wants. Checked for vacuum leaks at the boots? Pods? non stock exhaust? How are you determining it is lean exactly?

 

[JerrodR]

It has the Factory Airbox with a 4 into 1 into a Straight Pipe. It Ran great for Year's, so I just put everything back the way it was after the Swap. But when I Realized how Lean it was I put an old Muffler off a Mac System on it. Didn't seem to make much difference, nothing does.

 

[JerrodR]

http://www.hondachopper.com/garage/69-78_model_id_chart/69-78_model_id_chart.html

Interesting. The Head I bought didn't come with Hardware. So along with the Head I replaced the Rocker Tower's and Cam. Then took the Valve's and Spring's from the Original Head and installed them in the New. Everything seemed to Seat fine.

 

[JerrodR]

http://www.hondachopper.com/garage/69-78_model_id_chart/69-78_model_id_chart.html

From looking at this Chart My Original Head is not an F2. It has non-F2 Piston's as well. Their not the flat one's, Their the Bottom non-F2 Piston's with the little dome in the center. I hope this makes sense lol.

 

[JerrodR]

From looking at this Chart My Original Head is not an F2. It has non-F2 Piston's as well. Their not the flat one's, Their the Bottom non-F2 Piston's with the little dome in the center. I hope this makes sense lol.

Look's like I need to Pop the Motor back out and make sure I didn't Buy an F2 Head.

 

[dirtdigger]

The f2 has larger valves so no way parts would have swapped. f2 head has a 410 cast into the top fin of the head by the carbs.

 

[dirtdigger]

When you swapped the valves into the different head did you have a valve job done? You cant just swap valves and expect them to seat correct unless you are really lucky. Have you checked compression lately?

 

[JerrodR]

Look's like I need to Pop the Motor back out and make sure I didn't Buy an F2 Head.

Wait, but if I'm Reading this Chart right which I'd like to think I am. I would not be able to Install non-F2 Valve's into an F2 Head.

 

[dirtdigger]

Only f2 valves will fit in f2 head, they are larger than the k head. The k valves would literally fall in the ports of the f2

 

[JerrodR]

When you swapped the valves into the different head did you have a valve job done? You cant just swap valves and expect them to seat correct unless you are really lucky. Have you checked compression lately?

Yes, They were all between 140 and 150 psi.

 

[dirtdigger]

thats ok compression. How are you determining its lean? Double check for vacuum leaks at the head to carb boots.

 

[dirtdigger]

As I tell everyone, easiest and fastest way to jet is using the choke. When riding get to the trouble spot and hold the throttle, start closing the choke, if the engine picks up then its lean, if it stays about the same or bogs down its already too rich. These carbs are harder to jet because you have to use shims to change the needle position. Many people only try pilot and main jets which is a mistake. The needle jet has more affect of more of the throttle range than either of the fixed jets in the carb.

 

[netwurk13]

Could the head be cracked through any intake passages?

 

[JerrodR]

Could the head be cracked through any intake passages?

thats ok compression. How are you determining its lean? Double check for vacuum leaks at the head to carb boots

thats ok compression. How are you determining its lean? Double check for vacuum leaks at the head to carb bootsPlug Chop

thats ok compression. How are you determining its lean? Double check for vacuum leaks at the head to carb boots.

Plug's are Light Grey almost White. And the way it hesitates.

 

[JerrodR]

thats ok compression. How are you determining its lean? Double check for vacuum leaks at the head to carb boots.

 

[JerrodR]

This is Number 4. I've never seen anything like that before.

 

[JerrodR]

As I tell everyone, easiest and fastest way to jet is using the choke. When riding get to the trouble spot and hold the throttle, start closing the choke, if the engine picks up then its lean, if it stays about the same or bogs down its already too rich. These carbs are harder to jet because you have to use shims to change the needle position. Many people only try pilot and main jets which is a mistake. The needle jet has more affect of more of the throttle range than either of the fixed jets in the carb.

I have the Pumper Carb's. The Needle Position can be adjusted with a Wrench and Screwdriver kinda like You adjust the Valve Tapit's.