Iowa Urban Express doing low 40s with performance exhaust, SHA carb, BX31 belt – click on CC for captions.
I’ve seen many theories on how Honda restricted the Iowa Urban Express to limit its top speed. For that matter, I’ve seen many guesses at what the limited top speed is for the Iowa Urban Express – probably anywhere from 15mph to 25 mph. My research has led me to the conclusion that Honda limited the top speed to 25 mph. That is not a definitive answer but it is my best educated guess. Why did Honda name it the Iowa model when there were other states that also limited top speed on mopeds? I’m not sure. Maybe Iowa was the first state to do so. Anyhow, let’s get on to the bigger question of what Honda may have done to limit top speed on the Urban Express and what you can do about it.
I lied. Let’s deal with one more issue before we address the “bigger questions”. Remember you are dealing with a bike that is almost 40 years old. The biggest limiting factor in the top speed and overall performance of your bike is going to be from a lack of maintenance and care. Your Urban Express has probably been sitting in some garage (if you are lucky) or out in the elements (if you are not lucky) and has rotted away for several years.
I should repeat what I just said so you get it:
The biggest limiting factor in the top speed and overall performance of your Iowa Urban or regular Urban is going to be from a lack of maintenance and care.
I have bought two REGULAR Urban Expresses and both did about 25-27 mph when I first got them because of lack of maintenance and care.
A non-exhaustive list of maintenance that should be performed follows:
1. Clean the carburetor (you may have to clean it several times with special attention to idle jet and needle seat);
2. Replace fuel line;
3. Add new paper element fuel filter;
4. Clean the inside of the gas tank;
5. Replace the drive belt;
6. Replace air filter element;
7. Change the transmission oil;
8. Change the spark plug with proper plug with proper gap;
9. Replace the brake shoes;
10. Install new tires;
11. Tighten the spokes;
12. Check piston/rings/cylinder for scoring, blow-by, etc.;
13. Check head gasket, base gasket, exhaust gasket for possible improper sealing;
14. Check function of oil pump;
15. Etc., etc., etc.
Theories I’ve seen on Honda’s limitations on the Iowa edition.
Honda put a smaller jet in the Iowa edition when compared to the regular Urban
Rumor has it that Honda put a 75 or so main jet in the Iowa model and used an 85 jet in the regular Urban. My Iowa model came with an 85 jet. Now, the previous owner could have replaced that jet. I doubt it and here’s why. Honda over-jetted the regular Urban when it put in the 85 jet. I can speculate as to why but maybe later. But the point here is that by over-jetting the regular Urban Express, Honda limited it. Over-jetting leads to four stroking (I had some joker comment on one of my youtube videos basically saying that I was an idiot because two strokes can never four stroke. I think some village is still searching for you, Raymond J). Four stroking usually results in a loss of top speed and overall poorer performance.
If Honda wanted to limit the Iowa model, they would have just left the 85 jet in it. I think a 75 main jet would have actually led to better performance in either Urban edition. A 75 main jet would probably be too lean anyways. Somewhere around a 78 or 80 main jet would be perfect for a stock Urban, Iowa or otherwise.
Conclusion: Honda did not use a 75 main jet because it was probably too lean a main jet for the Iowa model or, it it wasn’t too lean, it would have actually increased top speed.
Honda put some restriction in the exhaust
Rumor has it that Honda hid some sort of restrictive device in the exhaust on Iowa models.
I don’t believe it. I took the exhaust off my Iowa model and put it on a regular stock Urban and got it to 34 mph no problem. Again, I don’t know the history of the bike and some former owner could have switched the exhaust, but, if memory serves, I could barely get the Iowa Urban to scare 25 mph when I first got it (which, in large part, is attributable to lack of maintenance). I didn’t do a thorough inspection of that exhaust but I did not notice anything different about it when compared to a regular Urban exhaust.
Another rumor claims that Honda used a different cylinder on the Iowa model.
I’ve only had one Iowa model but this jives with what I found and what I believe. The exhaust port appeared to be the small square exhaust port that I’ve seen in some NC50 bikes. Honda made at least two different cylinders for the Express and they probably did the same for the Urban. I’m pretty sure that the Urban and the Express share the same top end. One cylinder was the more restrictive one with the small square exhaust port and the other had a larger more rounded exhaust port.
Honda put a shorter, thinner drive belt on the Iowa model
I’m merely speculating here but my drive belt was either extremely worn or Honda used a smaller belt on the Iowa model.
Quick and dirty basic things you can do to get your Iowa (or any Urban up to speed)
In addition to the basic maintenance mentioned above,
1. After cleaning the carburetor, get a range of Keihin jets from 78-82. I believe they come in increments of two. Start with the 82 and work your way down, as necessary, to determine the best jetting.
2. Replace your air filter element with a less restrictive material.
3. Put a performance exhaust on (you will increase size of main jet back to the 85 or beyond with this although I was successful with an 82 main jet and MLM People’s Pipe exhaust).
4. Replace cylinder jug with one that has bigger exhaust port or take a dremel and open up the exhaust port on your current one (beginner’s beware – not for the inexperienced).
5. Install a new AX31 drive belt or a BX31 drive belt if you don’t mind loss of acceleration.
6. Or take an AX31 drive belt and flip it inside out and then install.
7. Adjust the rollers in your variator to get the best compromise between acceleration and top speed.
8. Shave down the cylinder head to increase compression.
9. Install a new piston and rings (as well as gaskets) and hone the existing cylinder.
10. Sounds stupid but air up the tires. I run close to 40 psi in mine, no problem. The ride is a little rougher but inadequate tire pressure will negatively affect top speed quite a bit.
11. I haven’t experiment with it much but you can play with the racing, blue CDI. I think it advances the timing a little bit.
12. Run premix instead of using the oil injection system. My feeling is that the oil injection system adds too much oil to the mix and leads to more four stroking. You will get better performance from premix at about a 40:1 or 50:1 ratio of gas to oil.
THINGS NOT TO DO
1. Don’t try to modify the existing exhaust. You will more likely ruin it than improve it. Sell the stock exhaust on eBay and take the money to buy a real performance exhaust.
2. Don’t run a super cold plug – that will lead to more four stroking. Right, Raymond J?
3. Don’t run without an air filter. You’re asking for trouble.
4. In the same vein, don’t run without a fuel filter.
5. Don’t try to remove the governor because there isn’t one.
Hope this helps!